[2][3] After he earned a master's degree in history from University of Nantes,[4][5] he became a permanent staff member of the Young Christian Workers (YCW) in 1991.

[3] Berger was employed by a social insertion association in Saint-Nazaire to help long-term unemployed adults and RMI beneficiaries to find jobs.

[1] On 17 June 2009 he was elected to the Confederal Executive Commission, the leading body of the CFDT,[9] where he was in charge of small business files.

[14] In 2016, Berger supported the Socialist government's labour law reform bill, in contrast with the FO and CGT unions.

On 11 December 2018 Berger tweeted that the CFDT had become the first trade union in France, overtaking the General Confederation of Labour (CGT).

"[18] In April 2017, during the 2017 French presidential election, Berger announced he had "clearly taken a stance against Marine Le Pen and published arguments for [CFDT's] militants", even if he did not give any voting instructions.