[1][4] Deffontaines began studying law and political science, as well as education, and took on multiple odd jobs.

He was then a member of a homework help association for young people in difficulty in sensitive areas of the city of Amiens and intended to become a teacher.

[5] His first years of communist activism were marked by the struggle of employees at the Goodyear site in Amiens, whose closure in 2014 led to the dismissal of more than 1,100 people, and the mobilizations against the reforms of François Hollande.

[8][9] On 1 July 2023, he was designated, by the national council of the PCF, the lead candidate for the communists for the 2024 European Parliament election.

[3] In the 2024 French legislative election, he was the candidate of the New Popular Front in Somme's 3rd constituency.