Originally from Haute-Savoie, Guillaume Cottet-Dumoulin was awarded a First Prize in euphonium, as well as a First Prize in Chamber Music, at the Paris Conservatoire (CNSM) in 1996.
At the same time, he studied the trombone and was also awarded a First Prize in 2000 at the Paris Conservatoire (CNSMDP).

He joined the Orchestre de Paris as principal trombonist in January 2001 and has performed under the baton of leading conductors such as Christoph Eschenbach, Paavo Järvi, Daniel Harding, Klaus Mäkelä, Esa-Pekka Salonen, Lahav Shani and Riccardo Chailly...

In 2003, he was a prize-winner at the Lieksa International Trombone Competition. He is a member of the Orchestre de Paris Brass Quintet, the Gilles Millière Trombone Quartet and the Timetracks jazz trio, ensembles with which he has made several recordings.

Passionate about teaching, he has taught the trombone at the CRR in Versailles and the CRR in Paris. In 2022, he was appointed professor of trombone at the Conservatoire National Supérieur de Musique et de Danse de Paris.

He performs regularly in France and abroad, where he also gives masterclasses.