[2] Quentin played in his first ITF Junior Circuit tournament in 2010 at the G4 Tournoi International de Clermont-Ferrand, as a wildcard.

In an all-French final, Halys and Armel Rancezot lost in the super tiebreak against Julien Delaplane and Alexandre Favrot.

[5][6] He streak was sniped by Belgian Clement Geens, at AEGON Junior International Nottingham, a 2-week G4 tournament.

[9] Starting 2012, Quentin played in all Junior Grand Slams but Wimbledon, where he didn't pass the third round of any of them, in singles.

Halys only final that year was at the GA Copa Gerdau, partnering Pedro Cachin, where they lost in straight sets to the partnership of Luke Bambridge and Joshua Ward-Hibbert.

[10] He played a second year for his country at the Junior Davis Cup, this time finishing in the fourth place.

All other finals were at G1 tournaments, winning only at the 35° Torneo International Citta Di Santa Croce, partnering Benjamin Bonzi.

Partnering Johan Sébastien Tatlot, the pair reached the final of the Australian Open, where they lost in straight sets.

He also entered as a wildcard in the 2016 Australian Open main draw where he defeated Ivan Dodig.

At the 2022 French Open as a direct entry, he lost to 23rd seed John Isner in four tight sets.

On his debut at the 2022 Wimbledon Championships he won his first match at this Major defeating his compatriot Benoît Paire.

Halys reached the quarterfinals of an ATP tournament for the first time in his career in Auckland, after beating Alex Molčan and Ben Shelton.

[22] Halys reached the third round at the 2023 Wimbledon Championships with wins over Dan Evans and Aleksandar Vukic for the first time at a Major.