Offer Letter, dated September 5, 2023, by and between Mersana Therapeutics, Inc. and Martin Huber

I am pleased to offer you the position of President and Chief Executive Officer (“CEO”) of Mersana Therapeutics, Inc. (the “Company”), and present you with the terms and conditions of your employment by the Company, as set forth in this letter agreement (this “Agreement”).

Position. As CEO, you will be responsible for such duties as are consistent with such position, plus such other duties as may from time to time be assigned to you by the Company, and you shall report to the Board of Directors of the Company (the “Board of Directors”). So long as you are employed by the Company as its CEO, the Board of Directors shall re-nominate you at each applicable annual meeting to serve as a member of the Board of Directors, with no additional remuneration payable for such service. Upon the ending of your employment as CEO, if so requested in writing by the Company, you shall immediately resign from the Board of Directors as well as from your position as CEO and any other position(s) with the Company to which you were elected or appointed in connection with your employment or Board of Directors’ membership. As a full-time employee of the Company, you will be expected to devote your full business time and energies to the business and affairs of the Company and shall not engage in any other employment, consulting or other business activity with any third party without the prior consent of the Board of Directors. Your performance will be reviewed on an annual basis.

You will work out of the Company’s office in Cambridge, Massachusetts, with the understanding that you will be required to travel to other locations in connection with the performance of your duties, at the expense of the Company.

Start Date and Nature of Relationship. Your start date is expected to be September 11, 2023. Your employment with the Company will be for no specified period and will constitute “at-will” employment. As a result, either you or the Company may terminate your employment relationship at any time and for any reason. No provision of this Agreement shall be construed to create an express or implied employment contract between you and the Company for any specific period of time.

Base Salary. Your initial base salary will be $26,041.67 per pay period (currently twice per month), which is $625,000 on an annualized basis and will be payable in accordance with the Company’s standard payroll schedule and less applicable taxes and withholdings. Your base salary may be adjusted from time to time in accordance with normal business practices and in the sole discretion of the Board of Directors (or a duly authorized committee thereof).

Annual Bonus. You will be eligible for an annual discretionary performance bonus with a target of 60% of your annual base salary, subject to the achievement of performance goals determined

by the Board of Directors (or a duly authorized committee thereof). The amount, terms and conditions of any annual bonus will be determined by the Board of Directors (or a duly authorized committee thereof) in its discretion, subject to the terms and conditions of any applicable bonus plan in effect from time to time and, for 2023, shall be pro-rated based on your start date. You must be an active employee of the Company on the date any bonus is distributed in order to be eligible for and to earn a bonus. The Company will award and pay any bonus for a calendar year before March 15th of the next calendar year.

Stock Options. Subject to approval by the Board of Directors (or a duly authorized committee thereof), the Company will grant to you an option to purchase 1,000,000 shares of the Company’s common stock, which option will be granted to you pursuant to the Mersana Therapeutics, Inc. 2017 Stock Incentive Plan (the “Plan”). The options will be subject to all of the terms and conditions set forth in the Plan, the option agreement governing the option, and the Company’s standard practices with regard to grants of this nature. These documents will be provided to you at the time the stock option is granted to you. In the event of any conflict between this letter and the Plan or the stock option agreement, the Plan and the stock option agreement will control.

Restricted Stock Units. Subject to approval by the Board of Directors (or a duly authorized committee thereof), the Company will grant to you 667,000 restricted stock units (“RSUs”), which RSUs will be granted to you pursuant to the Plan. The RSUs will be subject to all of the terms and conditions set forth in the Plan, the restricted stock unit agreement governing the RSUs, and the Company’s standard practices with regard to grants of this nature. These documents will be provided to you at the time the RSUs are granted to you. In the event of any conflict between this letter and the Plan or the restricted stock unit agreement, the Plan and the restricted stock unit agreement will control.

Benefits. You will be entitled to receive such benefits as are generally provided by the Company to its employees and for which you are eligible in accordance with Company policy and the terms and conditions of the applicable benefit plans, in each case, as in effect from time to time. The Company retains the right to change, add or cease any particular benefit at any time. You initially will be eligible for 12 paid holidays and 4 weeks’ paid vacation per year, which vacation eligibility will accrue at a rate of 1.67 days per month of service. You will be subject to the Company’s personnel policies in effect from time to time. The Company shall reimburse you for all reasonable and necessary documented out of pocket expenses incurred or paid by you in connection with, or related to, the performance of your services to the Company. You shall abide by the Company’s expense reimbursement policy, except as otherwise set forth herein or with the prior written approval of the Chairman of the Board.

Severance. In the event that your employment is terminated by the Company other than for Disqualifying Conduct (as defined below) (and not as a result of your death or disability) or you resign for Good Reason (as defined below) the Company shall, for twelve (12) months following the date your employment terminates, (i) continue to pay you your base salary as in effect on the date of termination (or, to the extent such base salary was reduced giving rise to Good Reason hereunder, as in effect immediately prior to such reduction), in accordance with its standard payroll procedures, and (ii) provided that you timely elect and are eligible to continue coverage in the Company’s group health plans in accordance with COBRA or applicable state law, pay a portion of the COBRA or applicable state law premium contributions on your behalf equal to the excess of the cost of such premiums for you, your spouse and dependents (if applicable) over the amount that you would have paid for such coverage had you remained continuously employed by the Company, at the level you were participating as of the date your employment terminates, in each case, subject to your signing and returning to the Company (and

not subsequently revoking, as applicable), within 60 days following the date on which your employment terminates or such shorter period as may be directed by the Company, an effective separation agreement in the form provided by the Company (which separation agreement shall include a release of claims and restrictive covenants substantially similar to those contained in the Confidentiality Agreement (as defined below), and an agreement not to compete with the Company for twelve (12) months following your separation from employment) (the “Separation Agreement”) and your continued compliance with the Confidentiality Agreement. Notwithstanding the foregoing, if the Company determines that its payment of the COBRA or applicable state law premium contributions would subject the Company to any tax or penalty, then this benefit shall not be provided.. Severance pay will be paid ratably in accordance with the Company’s regular payroll practices beginning in the Company’s first regular payroll cycle after the Separation Agreement becomes effective; provided, however, that if the 60th day referenced above occurs in the calendar year following the date of your termination, then the severance pay shall begin no earlier than January 1 of such subsequent calendar year.

For all purposes of this Agreement:

    “Disqualifying Conduct” shall mean, as determined by the Company: (i) willful misconduct or gross negligence as to a material matter in connection with your duties; (ii) any act or omission constituting material dishonesty or fraud with respect to the Company; (iii) the indictment for, conviction of, or a plea of guilty or nolo contendere to, a felony under applicable law; (iv) material violation of a material term of any written Company policy made available to you; (v) failure to attempt in good faith to (A) perform your duties in all material respects or (B) follow a clear, lawful and reasonable directive of the Board of Directors; or (vi) material breach of a fiduciary duty owed to the Company that has caused or could reasonably be expected to cause a material injury to the business; provided, that in no event shall your employment be terminated by reason of Disqualifying Conduct unless (A) an event or circumstance set forth in clauses (i), (ii), (iv) or (v) has occurred and the Company provides you with written notice after the Company has knowledge of the occurrence of existence of such event or circumstance, which notice reasonably identifies the event or circumstance that the Company believes constitutes Disqualifying Conduct and (B) with respect to the events and circumstances set forth in clause (v) only, you fail to substantially cure the event or circumstance so identified within 30 days of the receipt of such notice; and

    “Good Reason” shall mean, without your consent: (i) a material decrease in your base salary; (ii) a material diminution in your authorities, duties or responsibilities, or (iii) the relocation of your principal work location to a location more than 50 miles from its current location; provided, in each case, that (A) you provide written notice to the Company, setting forth in reasonable detail the event or events giving rise to Good Reason within 30 days following the initial occurrence of such event, (B) such event or events are not cured by the Company within a period of 30 days following its receipt of such written notice, and (C) you actually terminate your employment not later than 30 days following the expiration of such cure period.

Change in Control. In the event your employment is terminated by the Company other than for Disqualifying Conduct (and not as a result of your death or disability) or you resign for Good Reason, in each case, on or within 12 months following the consummation of a Change in Control (as defined below), in lieu of the payments set forth in the section entitled “Severance”

above, (i) the Company shall pay you a lump sum cash severance payment equal to the sum of (A) eighteen (18) months’ of your base salary and (B) 1.5 times your annual target bonus, in each case as in effect on the date of termination (or, to the extent such base salary was reduced giving rise to Good Reason hereunder, as in effect immediately prior to such reduction), (ii) for a period of eighteen (18) months following the date your employment terminates and provided that you timely elect and are eligible to continue coverage in the Company’s group health plans in accordance with COBRA or applicable state law, the Company shall pay a portion of the COBRA or applicable state law premium contributions on your behalf equal to the excess of the cost of such premiums for you, your spouse and dependents (if applicable) over the amount that you would have paid for such coverage had you remained continuously employed by the Company, at the level you were participating as of the date your employment terminates, and (iii) all of your stock options and other equity-based awards that vest solely based on the passage of time, to the extent outstanding immediately prior to the termination of your employment, will be treated as having vested in full as of immediately prior to such termination of employment, in each case, subject to your signing and returning to the Company (and not subsequently revoking, as applicable), within 60 days following the date on which your employment terminates or such shorter period as may be directed by the Company, an effective Separation Agreement in the form provided to you by the Company and your continued compliance with the Confidentiality Agreement. Notwithstanding the foregoing, if the Company determines that its payment of the COBRA or applicable state law premium contributions would subject the Company to any tax or penalty, then the Company may elect to pay to you in any month, in lieu of making such payments on your behalf, a cash payment equal to the Company’s cost of the monthly premium contribution for that month. Any cash severance payment made under this section entitled “Change in Control” will be paid in accordance with the Company’s regular payroll practices beginning in the Company’s first regular payroll cycle after the Separation Agreement becomes effective; provided, however, that if the 60th day referenced above occurs in the calendar year following the date of your termination, then the severance pay shall begin no earlier than January 1 of such subsequent calendar year.

For all purposes of this Agreement, the term “Change in Control” shall mean, as determined by the Company, a “change in control event” as that term is defined in the regulations under Section 409A of the Internal Revenue Code of 1986, as amended (the “Code”).

Confidentiality. The Company considers the protection of its confidential information and proprietary materials to be very important. Therefore, as a condition of your employment, you and the Company will become parties to a Nondisclosure and Assignment of Intellectual Property Agreement in the form of Attachment A to this Agreement (the “Confidentiality Agreement”). You acknowledge that your receipt of the grant of equity set forth in the sections entitled “Restricted Stock Units” and “Stock Options” above is contingent upon your agreement to the non-competition provisions set forth in the Confidentiality Agreement. You further acknowledge that such consideration was mutually agreed upon by you and the Company is fair and reasonable in exchange for your compliance with such non-competition obligations. Notwithstanding anything to the contrary in this Agreement, in the event you breach any provision of the Confidentiality Agreement or Separation Agreement (to the extent one arises as provided herein), the Company’s obligation to pay or provide, or continue to pay or provide, any salary continuation, severance or other benefits under the sections entitled “Severance” and “Change in Control” of this Agreement, as applicable, shall immediately cease.

Withholding. All payments made under this Agreement shall be reduced by any tax or other amounts required to be withheld by the Company, its successors or any of their respective affiliates under applicable law.

Section 409A. This Agreement is intended to comply with, or be exempt from, Section 409A of the Code and shall be interpreted accordingly. Notwithstanding anything to the contrary in this Agreement, if at the time your employment terminates, you are a “specified employee” (as defined below), any and all amounts payable under this Agreement on account of such separation from service that would (but for this provision) be payable within 6 months following the date of termination, shall instead be paid on the next business day following the expiration of such 6-month period or, if earlier, as soon as practicable, to the extent permitted under Section 409A of the Code, following your death; except to the extent of amounts or benefits that are not subject to the requirements of Section 409A of the Code. For purposes of this Agreement, all references to “termination of employment” and correlative phrases shall be construed to require a “separation from service” (as defined in Section 1.409A-1(h) of the Treasury regulations after giving effect to the presumptions contained therein), and the term “specified employee” means an individual determined by the Company to be a specified employee under Section 1.409A-1(i) of the Treasury regulations. Each payment made under this Agreement shall be treated as a separate payment and the right to a series of installment payments under this Agreement is to be treated as a right to a series of separate payments. Neither the Company nor you will have the right to accelerate or defer the delivery of any such payments except to the extent specifically permitted or required by Section 409A. In no event shall the Company have any liability relating to the failure or alleged failure of any payment or benefit under this Agreement to comply with, or be exempt from, the requirements of Section 409A of the Code.

Section 280G. If all, or any portion, of the payments or benefits provided under this Agreement, either alone or together with any other payment or benefit which you receive or are entitled to receive, would constitute an “excess parachute payment” within the meaning of Section 280G of the Code, then, notwithstanding anything in this Agreement or any other agreement or plan to the contrary, you shall be entitled to receive: (A) the amount of such payments or benefits, reduced such that no portion thereof shall fail to be tax deductible under Section 280G of the Code (the “Limited Amount”), or (B) if the amounts otherwise payable hereunder and under any other agreement or plan (without regard to clause (A)), reduced by all taxes applicable thereto (including, for the avoidance of doubt, the excise tax imposed by Section 4999 of the Code), would be greater than the Limited Amount reduced by all taxes applicable thereto, the amounts otherwise payable to you hereunder and under any other agreement or plan. All determinations under this section entitled “Section 280G” shall be made by an accounting, consulting, law or valuation firm selected, and paid for, by the Company.

At-Will Employment. This Agreement shall not be construed as an agreement, either expressed or implied, to employ you for any stated term, and shall in no way alter the Company’s policy of employment at will, under which both you and the Company remain free to terminate the employment relationship, with or without cause, at any time, with or without notice. Similarly, nothing in this Agreement shall be construed as an agreement, either express or implied, to pay you any compensation or grant you any benefit beyond the end of your employment with the Company, except to the extent set forth in the sections entitled “Severance” and “Change in Control” hereof.

Entire Agreement. Except as otherwise provided herein, this Agreement, together with the Confidentiality Agreement, constitutes the entire offer of employment between the parties and supersedes all prior and contemporaneous communications, agreements and understandings, written or oral, with respect to the subject matter hereof. No amendment to this offer will be permitted except in writing, signed by the parties hereto.

Governing Law. This Agreement shall be governed by the law of the Commonwealth of Massachusetts, without regard to any conflict of laws provisions.

Counterparts. This Agreement may be executed in two or more counterparts, each of which shall be an original and all of which together shall constitute one and the same instrument.

Contingent Offer. This employment offer is contingent upon your compliance with the following mandatory Company processes: (1) the COVID-19 Vaccination Process (included with this letter); and (2) a background check and references that are acceptable to the Company. In addition, this employment offer is contingent upon our verification of your identity and employment eligibility as required by federal law. Within 3 business days of your start of employment, you will be required to provide the Company with satisfactory proof of your identity and legal authorization to work in the United States in order to complete your Form I-9.

Representations. You represent and warrant that you are not bound by any employment contracts, restrictive covenants or other restrictions that would prevent you from carrying out your responsibilities for the Company, or which are in any way inconsistent with any of the terms of this Agreement. You acknowledge and represent that you have had the opportunity to fully discuss and review the terms of this Agreement with an attorney.

You may accept this offer of employment and the terms and conditions of this Agreement by signing this letter and the Confidentiality Agreement, which execution will evidence your agreement with the terms and conditions set forth herein and therein and returning them to the Company.

This offer of employment will expire at 5:00 PM Eastern Time, September 5, 2023, unless accepted by you prior to such date.