In 1977, he joined the Confédération générale du travail, and was put in charge of the union's "Young workers commission".

[1] In 1987, he joined the French Communist Party and, shortly thereafter, the CGT federal office for rail workers.

Thanks to his comparatively young age, and to the economic recovery in France at the end of the 20th century, he managed to counter the sag in CGT members.

The internal tensions in the Confédération Française Démocratique du Travail brought about by the Fillon retirement law prompted members to switch unions and reinforce the CGT.

However, Thibault's efforts to push the CGT towards a more reformist stance, as witnessed for instance during the strikes of November 2007, was met with resistance within the leadership of the union, which for example did not give a voting directive to its members for the 2005 referendum on the European Constitution.