Born in Brest in north-west France, Desmas played junior football for Ploumoguer before joining the youth setup at Stade Brestois at the age of 11.
[citation needed] At the end of the 2014–15 season, Desmas was offered a trial by Ligue 2 club Chamois Niortais, who were in search of a new goalkeeper following the departures of Rodolphe Roche and Lucas Bobe.
[2][3] During his first season at Niort, Desmas was third-choice goalkeeper behind Paul Delecroix and Saturnin Allagbé, and as such was restricted mainly to reserve-team action, making 17 appearances in the CFA2.
[4] Following the sale of Delecroix to Lorient in the summer of 2016, Desmas was promoted to second-choice goalkeeper behind Allagbé, with new signing Alexandre Bouchard taking over as third-choice.
[5][6] He went on to play in all five of the team's Coupe de France matches during the 2016–17 season,[7] a run which ended at the last-16 stage of the competition with a 0–2 home defeat to French champions Paris Saint-Germain thanks to late goals from Javier Pastore and Edinson Cavani.