Cover - USD ($)	12 Months Ended
	Dec. 31, 2022	Feb. 24, 2023	Jun. 30, 2022
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2022
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-38129
Entity Registrant Name	Mersana Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	04-3562403
Entity Address, Address Line One	840 Memorial Drive
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	498-0020
Title of 12(b) Security	Common Stock, $0.0001 par value
Trading Symbol	MRSN
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Public Float			$ 403,311,770
Entity Common Stock, Shares Outstanding		108,026,074
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement that will be filed for the 2023 Annual Meeting of Stockholders within 120 days of the end of the registrant’s fiscal year ended December 31, 2022 are incorporated by reference in Part III of this Annual Report on Form 10-K to the extent stated herein.
Entity Central Index Key	0001442836
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	FY
Amendment Flag	false