SUMMARY

        This summary highlights selected information from this document. It does not contain all of the information that may be important to you. You should carefully read this entire document and the other documents referred to in this document before you decide how to vote. Together these documents will give you a more complete description of the proposed transaction. Page references are included in this summary to direct you to more thorough descriptions of the topics provided elsewhere in these materials.

The Special Meetings of Shareholders and Stockholders

ACNB special meeting of shareholders to be held [    ·    ], 2019 (see page 47).

        ACNB will hold a special meeting of shareholders on [    ·    ], 2019, at [    ·    ]    .m., local time, at 100 V-Twin Drive, Gettysburg, Pennsylvania 17325.

FCBI special meeting of stockholders to be held [    ·    ], 2019 (see page 50).

        FCBI will hold a special meeting of stockholders on [    ·    ], 2019, at [    ·    ]    .m., local time, at [    ·    ], [    ·    ], Maryland [    ·    ].

ACNB record date set at [    ·    ], 2019; one vote per share of ACNB common stock (see page 47).

        If you owned shares of ACNB common stock at the close of business on [    ·    ], 2019, you are entitled to notice of, and to vote at, the special meeting of shareholders. You will have one vote at the special meeting for each share of ACNB common stock you owned on [    ·    ], 2019. On [    ·    ], 2019, there were [    ·    ] shares of ACNB common stock outstanding.

FCBI record date set at [    ·    ], 2019; one vote per share of FCBI common stock (see page 50).

        If you owned shares of FCBI common stock at the close of business on [    ·    ], 2019, you are entitled to notice of, and to vote at, the special meeting of stockholders. You will have one vote at the special meeting for each share of FCBI common stock you owned on [    ·    ], 2019. On [    ·    ], 2019, there were [    ·    ] shares of FCBI common stock outstanding.

The Companies

ACNB Corporation
16 Lincoln Square
Gettysburg, PA 17325
(717) 334-3161

        ACNB Corporation, with assets of $1.7 billion as of June 30, 2019, is the financial holding company for ACNB Bank and Russell Insurance Group, Inc. ACNB stock is quoted on The NASDAQ Capital Market under the symbol "ACNB." For more information, visit the ACNB website at www.investor.acnb.com. The information on ACNB's website is not incorporated into this joint proxy statement/prospectus.

Frederick County Bancorp, Inc.
9 North Market Street
Frederick, MD 21701
(301) 620-1400

        Frederick County Bancorp, Inc., with assets of $455 million as of June 30, 2019, is the financial holding company for Frederick County Bank. FCBI stock is quoted on the OTC Pink Market under the symbol "FCBI." For more information, visit the Frederick County Bank website at www.fcbmd.com. The

information on Frederick County Bank's website is not incorporated into this joint proxy statement/prospectus.

We propose that FCBI merge into a wholly-owned subsidiary of ACNB (see page 52).

        Under the terms of the reorganization agreement, ACNB will acquire FCBI by merging FCBI with and into a wholly-owned subsidiary of ACNB. FCBI will cease to exist as a separate entity. Following the merger, Frederick County Bank will merge with and into ACNB Bank. A copy of the reorganization agreement is attached to this document as Annex A and a copy of the bank plan of merger is attached to the reorganization agreement as Exhibit F.

FCBI stockholders will receive 0.9900 of a share of ACNB common stock for each share of FCBI common stock (see page 88).

        If the merger is completed, each share of FCBI common stock will be converted into the right to receive 0.9900 of a share of ACNB common stock. The exchange ratio will be appropriately adjusted if there is a stock dividend, stock split, reverse stock split, or common stock reclassification event regarding ACNB common stock before completion of the merger. FCBI stockholders will also receive cash instead of any fractional shares they would have otherwise received in the merger. ACNB estimates that it will issue a maximum of approximately 1,600,596 shares of ACNB common stock, $2.50 par value per share, in the merger, assuming the exercise of all outstanding options to purchase FCBI common stock.

        As a result, ACNB estimates that continuing ACNB shareholders will own approximately 81.5% of the issued and outstanding common shares of the combined company and that former FCBI stockholders will own approximately 18.5% of the issued and outstanding common shares of the combined company.

ACNB's board of directors has received an opinion from its financial advisor regarding the fairness of the exchange ratio to ACNB (see page 62).

        In connection with the merger, the board of directors of ACNB received a written opinion from ACNB's financial advisor, Boenning & Scattergood, Inc. ("Boenning"), as to the fairness, from a financial point of view and as of the date of such opinion, of the exchange ratio to ACNB. The full text of the opinion of Boenning, dated July 1, 2019, is included in this document as Annex B. ACNB encourages you to read this opinion carefully in its entirety for a description of the procedures followed, assumptions made, matters considered, and limitations of the review undertaken by Boenning. The opinion of Boenning is directed to ACNB's board of directors and does not constitute a recommendation to any shareholder of ACNB as to how to vote with respect to the issuance of shares of ACNB common stock in connection with the merger, or any other matter relating to the proposed merger.

FCBI's board of directors has received an opinion from its financial advisor regarding the fairness, from a financial point of view, of the merger consideration to be received by the holders of FCBI common stock (see page 76).

        In connection with the merger, the board of directors of FCBI received a written opinion from FCBI's financial advisor, Sandler O'Neill & Partners, L.P. ("Sandler O'Neill"), as to the fairness, from a financial point of view and as of the date of such opinion, of the merger consideration to be received by the holders of FCBI common stock. The full text of the opinion of Sandler O'Neill, dated July 1, 2019, is included in this document as Annex C. FCBI encourages you to read this opinion carefully in its entirety for a description of the procedures followed, assumptions made, matters considered, and limitations of the review undertaken by Sandler O'Neill. The opinion of Sandler O'Neill is directed to

FCBI's board of directors and does not constitute a recommendation to any stockholder of FCBI as to how to vote with respect to the merger or any other matter relating to the proposed merger.

ACNB shareholder vote required (see page 48).

        Approval of the Issuance of Shares.    In accordance with ACNB's bylaws, the affirmative vote of a majority of ACNB shares represented, in person or by proxy, at the special meeting is required to approve the issuance of shares of ACNB common stock in connection with the merger.

        Discretionary Authority to Adjourn or Postpone the Special Meeting.    In accordance with ACNB's bylaws, the affirmative vote of a majority of ACNB shares represented, in person or by proxy, at the special meeting is required to approve the proposal to adjourn or postpone the special meeting, if necessary, to solicit additional proxies in the event that there are not sufficient votes at the time of the special meeting to approve the issuance of shares of ACNB common stock in connection with the merger.

        Each holder of shares of ACNB common stock outstanding on the record date will be entitled to one vote for each share held of record. Brokers who hold ACNB common stock as nominees on your behalf will not have authority to vote your shares unless you provide voting instructions in accordance with the directions provided by your broker. Failure to provide your broker with voting instructions will have the same effect as a vote against the issuance of shares of ACNB common stock in connection with the merger.

ACNB directors and executive officers are expected to vote in favor of the issuance of shares (see page 48).

        The directors and executive officers of ACNB are expected to vote all shares of ACNB common stock that they own on the record date in favor of the issuance of the shares of ACNB common stock in connection with the merger. These persons own approximately [    ·    ] shares of ACNB common stock, or approximately [    ·    ]% of the outstanding shares of ACNB common stock.

FCBI stockholder vote required (see page 50).

        Approval and Adoption of the Reorganization Agreement.    The affirmative vote, in person or by proxy, of at least two-thirds of the shares of FCBI common stock outstanding on the record date is required to approve and adopt the reorganization agreement and the merger.

        Discretionary Authority to Adjourn or Postpone the Special Meeting.    In accordance with FCBI's bylaws, the affirmative vote of a majority of FCBI shares represented, in person or by proxy, at the special meeting is required to approve the proposal to adjourn or postpone the special meeting, if necessary, to solicit additional proxies in the event that there are not sufficient votes at the time of the special meeting to approve the proposal to approve and adopt the reorganization agreement and the merger.

        Each holder of shares of FCBI common stock outstanding on the record date will be entitled to one vote for each share held of record. The vote required for approval and adoption of the reorganization agreement is a percentage of all outstanding shares of FCBI common stock. Therefore, abstentions will have the same effect as a vote against the reorganization agreement and the merger. Brokers who hold FCBI common stock as nominees on your behalf will not have authority to vote your shares with respect to the reorganization agreement and the merger or the proposal to adjourn or postpone the FCBI special meeting unless you provide voting instructions in accordance with the directions provided by your broker. Failure to provide your broker with voting instructions will have the same effect as a vote against the reorganization agreement and the merger.

Certain current and former FCBI directors and executive officers have agreed to vote in favor of the merger (see page 108).

        On [    ·    ], 2019, the directors and executive officers of FCBI had sole or shared voting power over [    ·    ] shares, or [    ·    ]%, of the outstanding shares of FCBI common stock. These directors, certain executive officers of FCBI, and a former director and executive officer of FCBI having voting power over [    ·    ]% of the outstanding shares of FCBI common stock have agreed with ACNB to vote these shares of FCBI common stock in favor of approving and adopting the reorganization agreement and the merger.

ACNB and FCBI directors and executive officers may have interests in the merger that differ from your interests (see page 105).

        In considering the information contained in this joint proxy statement/prospectus, you should be aware that ACNB's and FCBI's directors and executive officers may have financial interests in the merger that are different from, or in addition to, the interests of ACNB shareholders or FCBI stockholders generally. These interests include, among other things:

•

William R. Talley, Jr., President and Chief Executive Officer of FCBI, will receive a consulting agreement for at least 18 months after the merger;



•

the right of certain executive officers to receive cash severance under their employment agreements with FCBI if their employment is terminated after the merger;



•

FCBI may award retention bonuses to certain employees;



•

Mr. Talley will become vested in an ongoing life insurance arrangement with a $100,000 death benefit;



•

the right to continued indemnification and liability insurance coverage for FCBI's current directors by ACNB after the merger for acts or omissions occurring before the merger; and,



•

the appointment of Kimberly S. Chaney, Chair of FCBI's board of directors, to ACNB's and ACNB Bank's boards of directors following completion of the merger, and any related compensation for such services.

        None of ACNB's executives and employees will receive change in control related payments or benefits since the merger does not constitute a "change in control" for purposes of ACNB's employment agreements, change in control agreements, or other agreements or plans.

        ACNB's and FCBI's board of directors was aware of these interests and considered them in approving and recommending the reorganization agreement. These and certain other additional interests of ACNB's and FCBI's directors and executive officers are described in detail in "Proposal 1—Issuance of ACNB Common Stock and the Merger—Interests of Directors and Executive Officers in the Merger" beginning at page 105. These circumstances may cause some of ACNB's and FCBI's directors and executive officers to view the proposed merger differently than you view it.

ACNB and FCBI must obtain regulatory approval and satisfy other conditions before the merger is complete (see page 99).

        ACNB's and FCBI's obligations to complete the merger and the bank merger are subject to various conditions that are usual and customary for this kind of transaction. These conditions include obtaining approval from the Federal Reserve Board, the FDIC, the Pennsylvania Department of Banking and Securities, and the Maryland Commissioner. As of the date of this document, appropriate applications for approval have been filed. In addition to the required regulatory approvals, the merger

will only be completed if certain conditions are met. See "Proposal 1—Issuance of ACNB Common Stock and the Merger—Terms of the Merger—Conditions to Merger."

Amendment or termination of the reorganization agreement is possible (see page 101).

        ACNB and FCBI can agree to amend the reorganization agreement in any way, except that, after approval by FCBI stockholders at its special meeting, ACNB and FCBI cannot change the amount of merger consideration FCBI stockholders will receive in the transaction from what is provided in the reorganization agreement.

        ACNB and FCBI may agree to terminate the reorganization agreement and not complete the merger at any time before the merger is completed. Each company also may unilaterally terminate the reorganization agreement in certain circumstances. This includes, among others, the failure to complete the merger by June 30, 2020, unless the terminating company's breach is the reason the merger has not been completed. See "Proposal 1—Issuance of ACNB Common Stock and the Merger—Terms of the Merger—Termination."

Rights of FCBI stockholders differ from those of ACNB shareholders (see page 166).

        When the merger is complete, FCBI stockholders who receive shares of ACNB common stock will become ACNB shareholders by operation of law. The rights of FCBI stockholders differ from the rights of ACNB shareholders in certain important ways. Many of these differences have to do with provisions in FCBI's articles of incorporation and bylaws that differ from those of ACNB. See "Comparison of Shareholders' Rights."

Material U.S. federal income tax consequences of the merger (see page 109).

        The merger is intended to qualify as a tax free "reorganization" within the meaning of Section 368(a) of the Code. Accordingly, FCBI stockholders generally will not recognize any gain or loss for U.S. federal income tax purposes on the exchange of shares of FCBI common stock for shares of ACNB common stock in the merger, except with respect to any cash received in lieu of fractional shares of ACNB common stock. FCBI stockholders will recognize gain or loss in connection with cash received in lieu of fractional shares of ACNB. This tax treatment may not apply to all FCBI stockholders.

        It is a condition to the closing of the merger that ACNB receive the opinion of its special counsel, Bybel Rutledge LLP, and that FCBI receive the opinion of its special tax counsel, Miles & Stockbridge P.C., substantially to the effect that, on the basis of facts, representations and assumptions set forth or referred to in that opinion (including factual representations contained in certificates of officers of ACNB and FCBI), the merger will be treated as a reorganization within the meaning of Section 368(a) of the Code. The condition is waivable and, in such case, ACNB and FCBI will undertake to recirculate and resolicit if the condition is waived by either party and the change in the tax consequences is material.

        ACNB and FCBI urge you to consult your tax advisor for a full understanding of the specific tax consequences of the merger to you. Tax matters are very complicated and, in many cases, tax consequences of the merger will depend on your particular facts and circumstances. See "Proposal 1—Issuance of ACNB Common Stock and the Merger—Material U.S. Federal Income Tax Consequences of the Merger" beginning on page 109.

FCBI stockholders are entitled to appraisal rights (see page 113).

        FCBI stockholders may object to the merger and, upon complying with the requirements of Maryland law, receive cash in the amount of the fair value of their shares instead of shares of ACNB

common stock specified in the reorganization agreement. A copy of the section of the MGCL pertaining to objecting stockholders' rights of appraisal is attached as Annex D to this joint proxy statement/prospectus. You should read the statute carefully and consult with your legal counsel if you intend to exercise these rights. The failure to comply with the statute exactly will result in the loss of your rights as an objecting stockholder.

Maintenance of Frederick County Bank identity after the merger (see page 99).

        For at least two years after the effective time of the merger, the former Frederick County Bank branches will operate under the name "FCB Bank, a division of ACNB Bank" unless the board of directors of ACNB shall determine otherwise upon approval of at least 66% of the ACNB board of directors.

Market Price and Dividend Information

ACNB

        As of [    ·    ], 2019, there were [    ·    ] shares of ACNB common stock held by approximately [    ·    ] shareholders of record. The number of shareholders does not reflect the number of investors holding stock in nominee name through banks, brokerage firms, and others.

        ACNB common stock trades on The NASDAQ Capital Market under the symbol "ACNB."

        On July 1, 2019, the last full trading day before the public announcement of the execution of the reorganization agreement, and on [    ·    ], 2019, the latest practicable trading day before the printing of this document, the high, low and closing sales prices for ACNB common stock were as follows:

		July 1, 2019									[·], 2019
		High			Low			Closing			High			Low			Closing
ACNB Common Stock		$	38.95		$	37.41		$	38.59

        Holders of ACNB common stock are entitled to dividends as and when declared by the ACNB board of directors out of funds legally available for the payment of dividends.

FCBI

        As of [    ·    ], 2019, there were [    ·    ] shares of FCBI common stock held by approximately [    ·    ] shareholders of record.

        FCBI common stock is quoted on the OTC Pink Market maintained by OTC Market Groups, Inc. under the symbol "FCBI." Over-the-counter market quotations reflect inter-dealer prices, without retail mark-up, mark-down or commission and may not necessarily represent actual transactions. The OTC Markets Group is an electronic, screen-based market which imposes considerably less stringent listing standards than The NASDAQ Capital Market.

        On July 1, 2019, the last full trading day before the public announcement of the execution of the reorganization agreement, and on [    ·    ], 2019, the latest practicable trading day before the printing of this document, the high, low and closing sales prices for FCBI common stock were as follows:

		July 1, 2019									[·], 2019
		High			Low			Closing			High			Low			Closing
FCBI Common Stock		$	27.00		$	27.00		$	27.00

        Under the reorganization agreement, FCBI is prohibited from paying any dividend or distribution to its stockholders before the effectiveness of the merger, other than its regular quarterly dividend of

$0.08 per share, without the prior written consent of ACNB. FCBI's ability to pay dividends is also subject to state and federal laws and regulations.

Comparative Market Value

        The following table sets forth the market value per share of ACNB and FCBI common stock and the equivalent market value per share of FCBI common stock on July 1, 2019 (the last trading day prior to the date of the public announcement of the merger) and [    ·    ], 2019 (the latest practicable trading day prior to the date of this document). The equivalent market value is based upon the exchange ratio of 0.9900 of a share of ACNB common stock multiplied by the closing sales price of ACNB common stock on the specified date.

		ACNB Historical			FCBI Historical			FCBI Equivalent Market Value
July 1, 2019		$	38.59		$	27.00		$	38.20
[·], 2019		$			$			$

Adjournment or Postponement Proposal (Page 175)

        You are being asked to approve a proposal to grant ACNB and FCBI discretionary authority to adjourn or postpone the special meetings, if necessary, to solicit additional proxies in the event there are insufficient votes, in the case of ACNB, to approve the issuance of shares of ACNB common stock in connection with the merger and, in the case of FCBI, to approve and adopt the reorganization agreement and the merger.

COMPARATIVE HISTORICAL AND PRO FORMA PER SHARE DATA

        Presented below for ACNB and FCBI are comparative historical and unaudited pro forma equivalent per share financial data for the year ended December 31, 2018, and as of and for the six months ended June 30, 2019. The information presented below should be read together with the historical consolidated financial statements of ACNB and FCBI, including the related notes. The FCBI financial statements are included in this joint proxy statement/prospectus beginning on page F-1. The ACNB financial statements are incorporated by reference into this joint proxy statement/prospectus. See "Where You Can Find More Information" on page 176. The per share data is not necessarily indicative of the operating results that FCBI would have achieved had it completed the merger as of the beginning of the periods presented and should not be considered as representative of future operations.

        The unaudited pro forma information gives effect to the merger as if the merger had been effective on December 31, 2018 or June 30, 2019 in the case of the book value data, and as if the merger had been effective as of January 1, 2018 in the case of the earnings per share and the cash dividends data. The unaudited pro forma data combines the historical results of FCBI into ACNB's consolidated financial statements. While certain adjustments were made for the estimated impact of fair value adjustments and other acquisition-related activity, they are not indicative of what could have occurred had the acquisition taken place on January 1, 2018.

        The FCBI pro forma equivalent per share amounts are calculated by multiplying the ACNB pro forma combined book value per share, cash dividends per share and net income per share by the exchange ratio of 0.9900 so that the per share amounts equate to the respective values for one share of FCBI common stock. The unaudited pro forma ACNB per share equivalents are calculated by combining the ACNB historical share amounts with pro forma amounts from FCBI, assuming the exchange ratio of 0.9900.

        You should read the information below in conjunction with the selected consolidated historical financial data included elsewhere in this joint proxy statement/prospectus and the historical consolidated financial statements of ACNB and FCBI and related notes that have been filed with the SEC, certain of which are incorporated by reference herein. See "Selected Financial and Other Data of ACNB", "Selected Financial and Other Data of FCBI" and "Where You Can Find More Information" beginning on pages 17, 18 and 176, respectively. The unaudited pro forma combined per share data and the unaudited pro forma combined per share equivalent data has been derived from, and should be read in conjunction with, the unaudited pro forma condensed combined financial statements and related notes included in this joint proxy statement/prospectus. See "Unaudited Pro Forma Combined Consolidated Financial Data" beginning on page 20, which are based on and should be read in conjunction with (i) the historical audited consolidated financial statements of ACNB and the related notes included in ACNB's Annual Report on Form 10-K for the year ended December 31, 2018 and the historical unaudited interim consolidated financial statements and related notes included in ACNB's Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2019, each of which is incorporated by reference herein, and (ii) the historical audited consolidated financial statements of FCBI for the year ended December 31, 2018 and the historical unaudited interim consolidated financial statements and related notes included for the quarterly period ended June 30, 2019, each of which is included in this joint proxy statement/prospectus beginning on page F-1.

Unaudited Pro Forma Per Share Data
For The Six Months Ended June 30, 2019
($ in Thousands, Except Per Share Data)

		ACNB Historical			FCBI Historical			Pro Forma Combined(1)(2)(3)			Pro Forma Equivalent FCBI Share(4)
For The Six Months Ended June 30, 2019:
Earnings per share
Net income per share (Basic)		$	1.75		$	0.67		$	1.61		$	1.60
Net income per share (Diluted)		$	1.75		$	0.65		$	1.61		$	1.60
Cash Dividends Per Share		$	0.48		$	0.16		$	0.48		$	0.48
Book Value per common share as of June 30, 2019		$	25.59		$	23.51		$	27.49		$	27.22
Tangible Book Value per common share as of June 30, 2019		$	22.15		$	23.51		$	20.88		$	20.67

Unaudited Pro Forma Per Share Data
For The Twelve Months Ended December 31, 2018
($ in Thousands, Except Per Share Data)

		ACNB Historical			FCBI Historical			Pro Forma Combined(1)(2)(3)			Pro Forma Equivalent FCBI Share(4)
Earnings per share:
For the year ended December 31, 2018
Net income per share (Basic)		$	3.09		$	1.95		$	3.00		$	2.97
Net income per share (Diluted)		$	3.09		$	1.88		$	3.00		$	2.97
Cash Dividends Per Share		$	0.89		$	0.34		$	0.89		$	0.88
Book Value per common share as of December 31, 2018		$	23.86		$	23.06		$	25.91		$	25.65
Tangible Book Value per common share as of December 31, 2018		$	20.46		$	23.06		$	19.31		$	19.12

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(1)

The pro forma combined basic earnings and diluted earnings per share of ACNB common stock is based on the pro forma combined net income for ACNB and FCBI divided by the pro forma common shares or diluted common shares of the combined entities. The pro forma information includes adjustments related to the fair value of assets and liabilities of FCBI and is subject to adjustment as additional information becomes available and as a final merger date analyses are performed.

(2)

The pro forma earnings does not include anticipated cost savings or revenue enhancements, nor does it include one-time merger-related expenses which will be expensed against income. ACNB and FCBI currently anticipates potential annual pre-tax cost savings following the merger will be approximately $4.7 million, with 65% expected to be realized in year 1 after the merger, but there is no assurance that the anticipated cost savings will be realized on the anticipated time schedule or at all.

(3)

The pro forma combined book value and tangible book value per share data does include the impact of merger-related expenses. Such expenses are currently estimated to consist of FCBI after-tax charges currently of $6.5 million and ACNB after-tax estimated charges of $5.4 million.