UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): September 9, 2015

Farmers National Banc Corp.

(Exact name of registrant as specified in its charter)

(330) 533-3341

(Registrant’s telephone number, including area code)

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On September 9, 2015, Farmers National Banc Corp. (the “Company”) first utilized a revised investor presentation reflecting the Company’s financial results for the quarter ended June 30, 2015. The investor presentation also reviewed the Company’s previously announced proposed merger with Tri-State 1^st Banc, Inc. (“TSOH”), the parent company of 1^st National Community Bank (“FNCB”), pursuant to the Agreement and Plan of Merger (the “Merger Agreement”) among the Company, Tri-State, and FMNB Merger Subsidiary, LLC, a newly-formed wholly-owned subsidiary of the Company (the “Merger”).

A copy of the investor presentation is attached hereto as Exhibit 99.1 and incorporated herein by reference. The investor presentation is furnished herein, as part of this Item 7.01, as Exhibit 99.1. Pursuant to General Instruction B.2 of Current Report on Form 8-K, the information in this Item 7.01 and Exhibit 99.1 shall not be deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liability of that section. Furthermore, the information in this Item 7.01 and Exhibit 99.1 shall not be deemed to be incorporated by reference into the filings of the Company under the Securities Act of 1933, as amended (the “Securities Act”), or the Exchange Act, except as may be expressly set forth by specific reference in such filing.

On September 9, 2015, the Company first utilized a revised investor presentation reflecting the Company’s financial results for the quarter ended June 30, 2015, and reviewing the Merger.

Also on September 9, 2015, the Company and TSOH issued a press release announcing the election deadline for TSOH common shareholders to elect the form of consideration they wish to receive for their TSOH common shares in connection with the Merger. A copy of the press release is attached as Exhibit 99.2 to this Current Report on Form 8-K and incorporated herein by reference.

The press release is furnished herein, as part of this Item 8.01, as Exhibit 99.2. Pursuant to General Instruction B.2 of Current Report on Form 8-K, the information in this Item 8.01 and Exhibit 99.2 shall not be deemed to be “filed” for purposes of Section 18 of the Exchange Act, or otherwise subject to the liability of that section. Furthermore, the information in this Item 8.01 and Exhibit 99.2 shall not be deemed to be incorporated by reference into the filings of the Company under the Securities Act, or the Exchange Act, except as may be expressly set forth by specific reference in such filing.

Forward Looking Statements.

This report contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These statements are not historical facts, but rather statements based on the Company’s current expectations regarding its business strategies and its intended results and future performance. Forward-looking statements are preceded by terms such as “expects,” “believes,” “anticipates,” “intends” and similar expressions, as well as any statements related to future expectations of performance or conditional verbs, such as “will,” “would,” “should,” “could” or “may.”

Forward-looking statements are not guarantees of future performance. Numerous risks and uncertainties could cause or contribute to the Company’s actual results, performance, and achievements to be materially different from those expressed or implied by the forward-looking statements. Factors that could cause Farmers’ actual results to differ materially from those

described in the forward-looking statements can be found in Farmers’ Annual Report on Form 10-K for the year ended December 31, 2014, as amended, which has been filed with the Securities and Exchange Commission and is available on Farmers’ website (www.farmersbankgroup.com) and on the Securities and Exchange Commission’s website (www.sec.gov). Factors that may cause or contribute to these differences may also include, without limitation, the Company’s failure to integrate TSOH and its subsidiary in accordance with expectations; deviations from performance expectations related to TSOH and its subsidiary; general economic conditions, including changes in market interest rates and changes in monetary and fiscal policies of the federal government; legislative and regulatory changes; competitive conditions in the banking markets served by the Company’s subsidiaries; the adequacy of the allowance for losses on loans and the level of future provisions for losses on loans; and other factors disclosed periodically in the Company’s filings with the SEC.

Because of the risks and uncertainties inherent in forward-looking statements, readers are cautioned not to place undue reliance on them, whether included in this report or made elsewhere from time to time by the Company or on the Company’s behalf. The Company assumes no obligation to update any forward-looking statements.

The following exhibits are filed herewith:

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: September 9, 2015