He previously served as the Parliamentary secretary to the Leader of the Government in the House of Commons, a position he held effective December 3, 2021.

As Mayor, Gerretsen worked alongside the City of Kingston and municipal leaders across Ontario, advancing issues such as payments in lieu of taxes, the integration of social services, and increased investment in local infrastructure and affordable housing.

In his first term in federal office, Gerretsen served on the Standing Committee for National Defense, and was elected as Chair of the Ontario Liberal Caucus.

[8] Gerretsen's Private Members Bill, Bill C-243, The National Maternity Assistance Program Strategy, aimed to introduce amendments to the Employment Insurance Act to create a national maternity assistance program for women who are unable to work during their pregnancy and was based on the first-hand experience of a constituent in his riding.

As a strong environmental proponent, Gerretsen openly spoke against the Government's decision to expand the Kinder Morgan pipeline.

In June 2018, Mark was one of three Liberal MPs who voted in favour of a motion proposed by the New Democratic Party to stop the project.

[13] On September 19, 2024, Gerretsen issued a public retraction for spreading disinformation about commentator Kat Kanada and announced that he was making a personal donation to a charity chosen by her.

[15] Personally, Gerretsen was an early participant in the province of Ontario's Microfit program aimed at encouraging homeowners to install solar panels to feed electricity back into the grid.