UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): December 28, 2016

MUSTANG BIO, INC.

(Exact Name of Registrant as Specified in Charter)

Registrant’s Telephone Number, Including Area Code: (781) 652-4507

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:

Item 1.01. Entry into a Material Definitive Agreement.

The information set forth under Item 3.02 of this Current Report on Form 8-K regarding the Transaction Documents (as such term is defined in Item 3.02 below) is incorporated herein by reference.

Item 3.02. Unregistered Sales of Equity Securities.

On December 29, 2016, Mustang Bio, Inc. (the “Company”) completed a fifth closing of the private placement (the “Transaction”) previously reported by the Company in its Current Report on Form 8-K filed with the Securities and Exchange Commission on October 6, 2016 (the “Prior 8-K”), which is incorporated by reference herein. In this fifth closing, the Company sold 63.22 Units to certain accredited investors (the “Units”) pursuant to a Unit Purchase Agreement (each a “Purchase Agreement”). As described in the Prior 8-K, each Unit consists of 10,000 shares of the Company’s common stock, $0.001 par value per share (the “Common Stock”) and warrants (the “Warrants”) exercisable immediately for 2,500 shares of Common Stock for five years at an exercise price of $8.50 per share, for a purchase price of $65,000 per Unit. The Units sold in the fifth closing are comprised of 632,246 shares of Common Stock and Warrants to purchase 158,050 shares of Common Stock for aggregate gross proceeds of $4,109,599.00.

In connection with the fifth closing, the Company’s placement agent, pursuant to the Placement Agent Agreement dated August 3, 2016 (the “Placement Agent Agreement” and, as amended on December 6, 2016 by an Amendment to the Placement Agent Agreement, the “Amended Placement Agent Agreement”), received a cash fee of $410,959 and a warrant to purchase 63,225 shares of Common Stock (the “Placement Agent Warrant” and, collectively with the Purchase Agreement, Warrants, and Amended Placement Agent Agreement, the “Transaction Documents”).

The Units, Warrants and Placement Agent Warrant were offered and sold in a private placement pursuant to exemptions from the registration requirements of the Securities Act of 1933, as amended (the “Securities Act”), afforded by Section 4(a)(2) and Rule 506 of Regulation D promulgated thereunder. To the extent that any shares of Common Stock are issued in connection with the Transaction, they may not be offered or sold in the United States absent registration or the availability of an applicable exemption from the registration requirements of the Securities Act.

The foregoing descriptions of the Transaction Documents are subject to the full and complete terms of each such Transaction Document and the form of Purchase Agreement, form of Warrant, the Placement Agent Warrant and the Placement Agent Agreement which were filed with the Company’s Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 14, 2016, and the Amendment to the Placement Agent Agreement which will be filed with the Company’s Annual Report on Form 10-K for the year ended December 31, 2016.

Item 8.01. Other Events.

On December 28, 2016, the Company used a portion of the proceeds from the multiple closings of its private placement described in Item 3.02 to repay the promissory note in favor of NSC Biotech Venture Fund I, LLC dated February 27, 2015 in the original principal amount of $3,600,000.

The information contained in this Current Report on Form 8-K is not an offer to sell or the solicitation of an offer to buy any securities of the Company.

SIGNATURE

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 thereunto duly authorized.