(LREM) who was elected to the French National Assembly on 18 June 2017, representing the department of Haute-Saône.

In addition to his committee assignments, he is a member of the French-Belarusian Parliamentary Friendship Group.

[2] In November 2019, 20 members of the yellow vests movement tried to force open the door of Lejeune's house in the Haute-Saône.

[3] In July 2019, Lejeune decided not to align with his parliamentary group's majority and became one of 52 LREM members who abstained from a vote on the French ratification of the European Union's Comprehensive Economic and Trade Agreement (CETA) with Canada.

[4] In late 2019, Lejeune was one of 17 members of the Defense Committee who co-signed a letter to Prime Minister Édouard Philippe in which they warned that the 365 million euro ($406 million) sale of aerospace firm Groupe Latécoère to U.S. fund Searchlight Capital raised "questions about the preservation of know-how and France's defense industry base" and urged government intervention.