| Cover | Nov. 30, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 30, 2020 |
| Entity File Number | 001-38129 |
| Entity Registrant Name | MERSANA THERAPEUTICS, INC. |
| Entity Central Index Key | 0001442836 |
| Entity Tax Identification Number | 04-3562403 |
| Entity Incorporation, State or Country Code | DE |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.0001 par value |
| Trading Symbol | MRSN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | true |
