The following year he joined Chambéry CF [fr], the development team of AG2R La Mondiale after hitchhiking along the nine-hour drive to apply.

Cosnefroy completed his first full professional season in 2018, notably placing third in Paris–Tours and ninth in the Bretagne Classic Ouest-France, his first top ten in a UCI World Tour race.

[13] Cosnefroy took five victories in 2019, all in France: the overall classification and third stage of the Tour du Limousin as well as the one-day races Grand Prix de Plumelec-Morbihan, Paris–Camembert and Polynormande.

[14] At the beginning of 2020, he won the Grand Prix La Marseillaise and the Étoile de Bessèges before the season stopped due to the Covid-19 pandemic.

[15] As soon as racing resumed, he won the fourth and final stage of the Route d'Occitanie ahead of Bauke Mollema, Thibaut Pinot and Egan Bernal.

He ended the season strongly, placing second to Marc Hirschi in the La Flèche Wallonne, as well as third in De Brabantse Pijl and second in Paris–Tours.

Cosnefroy started 2024 on strong form, winning stage 2 and the overall title of the Tour des Alpes-Maritimes in February, followed up with a sixth place finish at Strade Bianche in early March.