UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of
the Securities Exchange Act of 1934 (Amendment No.     )

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MERSANA THERAPEUTICS, INC.
(Name of Registrant as Specified In Its Charter)
(Name of Person(s) Filing Proxy Statement, if other than the Registrant)
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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on June 12, 2020 MERSANA THERAPEUTICS, INC. You are receiving this communication because you hold shares in the above named company. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. MERSANA THERAPEUTICS, INC. 840 MEMORIAL DRIVE CAMBRIDGE, MA 02139 0000458856_1 R1.0.1.18 See the reverse side of this notice to obtain proxy materials and voting instructions. Meeting Information Meeting Type: Annual Meeting For holders as of: April 15, 2020 Date: June 12, 2020Time: 9:00 AM EDT Location: Meeting live via Audio Webcast - please visit www.virtualshareholdermeeting.com/MRSN2020

Before You Vote How to Access the Proxy Materials Proxy Materials Available to VIEW or RECEIVE:  Have the information that is printed in the box marked by the arrow (located on the  by the arrow (located on the following page) in the subject line. How To Vote Please Choose One of the Following Voting Methods possession  available and follow the instructions. marked by the arrow 0000458856_2 R1.0.1.18 Vote In Person: Many stockholder meetings have attendance requirements including, but not limited to, the of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any specia requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the information that is printed in the box Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. l 1. Form 10-K2. Notice & Proxy Statement How to View Online: following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER or E-MAIL Copy: If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET: www.proxyvote.com 2) BY TELEPHONE: 1-800-579-1639 3) BY E-MAIL*: sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before May 31, 2020 to facilitate timely delivery.

The Board of Directors recommends you vote FOR the following: 1. Election of Class III Directors Nominees 01 David Mott 02 Anna Protopapas 03 Lawrence M. Alleva The Board of Directors recommends you vote FOR the following proposal: 2To ratify the selection of Ernst & Young LLP as the independent registered public accounting firm for the Company for the fiscal year ending December 31, 2020. NOTE: Such other business as may properly come before the meeting or any adjournment thereof. 0000458856_3 R1.0.1.18 Voting items

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