[citation needed] Demers began his professional career with San Jose's American Hockey League (AHL) affiliate, the Worcester Sharks, scoring 33 points in 78 games in the 2008–09 season.

[citation needed] Demers made his NHL debut on October 1, 2009, against the Colorado Avalanche after surviving the final cut in the San Jose Sharks training camp for the 2009–10 season.

Demers scored his first career Stanley Cup playoff goal against the Chicago Blackhawks on May 16 in the Western Conference Finals, a series the Sharks lost in a four-game sweep.

[5] On July 2, 2016, Demers left the Stars organization as a free agent and signed a five-year deal worth $22.5 million with the Florida Panthers.

On December 20, 2021, Demers opted to continue his career abroad, agreeing to a one-year contract for the remainder of the season with Russian club Ak Bars Kazan of the KHL.

With aspirations to resume his career in the NHL, Demers accepted an invitation to join the Edmonton Oilers training camp and pre-season in preparation for the 2022–23 season.