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): May 28, 2020

MERSANA THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

(Registrant’s telephone number, including area code): (617) 498-0020

Not Applicable

(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:

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company x

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. x

On May 28, 2019, Mersana Therapeutics, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Cowen and Company, LLC and SVB Leerink LLC, as representatives of the several underwriters listed on Schedule 1 thereto (the “Underwriters”), relating to a public offering (the “Offering”) of 8,000,000 shares (the “Firm Shares”) of the Company’s common stock, $0.0001 par value per share (the “Common Stock”), at a price to the public of $19.00 per share. In addition, the Company granted the Underwriters an option to purchase an additional 1,200,000 shares (the “Optional Shares” and together with the Firm Shares, the “Shares”) of Common Stock at the same price per share as the Firm Shares. On May 29, 2020, the Underwriters exercised in full this option, which was exercisable for 30 days from the date of the prospectus supplement. The Shares were offered pursuant to a registration statement on Form S-3 (File No. 333-238140) and a related prospectus supplement, as well as a related registration statement on Form S-3MEF filed with the Securities and Exchange Commission pursuant to Rule 462(b) under the Securities Act of 1933, as amended. Subject to customary closing conditions, the closing of the Offering, including the sale of the Firm Shares and the Optional Shares, is expected to take place on June 2, 2020.

The above description of the Underwriting Agreement is qualified in its entirety by reference to the Underwriting Agreement, which is filed as Exhibit 1.1 hereto and incorporated herein by reference.

An opinion dated June 1, 2020, regarding the legality of the issuance and sale of the Shares in the Offering is filed as Exhibit 5.1 to this Current Report on Form 8-K. Certain information relating to Part II, Item 14 “Other Expenses of Issuance and Distribution” of the Registration Statement is filed as Exhibit 99.1 to this Current Report on Form 8-K.

(d) Exhibits.

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: June 1, 2020