Cormier was drafted 68th overall by the Golden Knights in the third round of the
2020 NHL entry draft, signing his three-year entry-level contract on December 31, 2020, and played junior hockey with the
Charlottetown Islanders of the QMJHL. Cormier experienced a breakout season in
2020–21 season, winning the
Emile Bouchard Trophy as the league's best defenseman, and being named to the season's
First All-Star Team. Cormier repeated both feats in
2021–22 season, additionally leading the league in both goals and points by a defenseman, and helping the Islanders to the
President's Cup Finals, where they ultimately lost to the
Shawinigan Cataractes. Cormier made his NHL debut with Vegas on January 6, 2024, recording an assist on a third-period
Jack Eichel power-play goal for his first NHL point. Cormier made his 2024–25 season debut in the AHL on March 1, 2025, following a long-term injury. Following the season, as a restricted free agent, Cormier signed a one-year extension with Vegas on July 8, 2025. ==International play==