[8] On March 26, 2018, Mittelstadt opted to leave the college ranks and was signed to a three-year, entry-level contract by the Buffalo Sabres.

[11] Rather than assign him to the American Hockey League (AHL), Buffalo management opted to have Mittelstadt play the entire 2018–19 season with the team.

With the Sabres outside playoff contention, Mittelstadt's seven year tenure ended after he was traded to the Colorado Avalanche in exchange for defenseman Bowen Byram on March 6, 2024.

[15] Acquired to assume second-line center duties behind Nathan MacKinnon, he made his debut two days later for the Avalanche in a 2-1 overtime victory over the Minnesota Wild on March 8, 2024.

[17] Adding needed depth to the Avalanche center position, Mittelstadt completed the regular season with 10 points through 18 games.

After the Avalanche's second-round defeat to the Dallas Stars, Mittelstadt as a restricted free agent was later re-signed by Colorado to a three-year, $17.25 million contract extension on June 26, 2024.