From 2009 until 2012, Saint-Martin was a member of the
Socialist Party. However, he was not actively involved in politics before he joined
La République En Marche! in 2016. In the
2017 legislative election, Saint-Martin was elected to the National Assembly, where he represented the
3rd constituency of
Val-de-Marne. He succeeded
Roger-Gérard Schwartzenberg of the
Radical Party of the Left. In Parliament, Saint-Martin served as a member of the Finance Committee. In addition to his committee assignments, he was part of the French-Peruvian Parliamentary Friendship Group. In late 2018, Saint-Martin was offered to join the
government of
Prime Minister Édouard Philippe but declined a post as Secretary of State at the
Ministry of the Economy and Finance under the leadership of
Bruno Le Maire. In June 2019, Philippe entrusted him with a mission to reform the national system for the
identification, seizure and confiscation of criminal assets. From 2020, Saint-Martin served as the Parliament's
lead rapporteur on the
annual budget of France; he succeeded
Joël Giraud. Within his party, Saint-Martin became a member of the executive board in 2019. In that capacity, he was entrusted alongside
Guillaume Chiche for the party's policy planning. In early 2021, Saint-Martin emerged as the frontrunner in the race to lead the La République En Marche! campaign in
Île-de-France during that year's
regional elections and to potentially succeed
Valérie Pécresse as President of the
Regional Council of Île-de-France. With only 9.62 percent of the vote, he ultimately lost against Pécresse but was elected as a regional councillor. In the
2022 legislative election, Saint-Martin ran for reelection to the National Assembly but
lost his seat to
Louis Boyard of
La France Insoumise. ==CEO of Business France, 2023–2024==