[4][3] After working in the office of Jean-Paul Huchon at the Regional Council of Île-de-France from 2012 to 2014, Séjourné became an adviser to Minister of the Economy and Finance Emmanuel Macron.

In addition to his committee assignments, Séjourné is part of the Parliament's delegations for relations with Mercosur and the Euro-Latin American Parliamentary Assembly.

[10] In May 2022, Séjourné – together with Patrick Mignola for the Democratic Movement (MoDem) and Gilles Boyer for Horizons – negotiated the agreement leading to the creation of Ensemble, a coalition of the parties forming the presidential majority, including on the financial distribution between them.

[19] In an article published by Le Journal du Dimanche in February 2021, Séjourné opposed the proposed Comprehensive Agreement on Investment (CAI).

[22] Séjourné is gay and was in a civil union (PACS) with Gabriel Attal,[23][24][25] a French politician who served as the Prime Minister of France in 2024.