On 15 April 2014, at the conclusion of his season with AIK, Janmark was assigned by the Red Wings to the Grand Rapids Griffins on an amateur try-out contract.

[4] At the conclusion of the season, having scored 36 points in 55 games, Janmark signed a two-year, entry-level contract with the Stars on 24 April 2015.

Janmark made his NHL debut on 8 October 2015, netting his first career goal on his first shot on his first shift, 99 seconds into the game.

[7] Having left the Stars organization as a free agent after four seasons, Janmark signed a one-year, $2.25 million contract with the Chicago Blackhawks on 12 October 2020.

[9] During the following playoffs, on 28 May 2021, he scored a hat trick for the Golden Knights in the seventh game of their first-round matchup against the Minnesota Wild.

A free agent from the Golden Knights following the 2021–22 season, Janmark signed a one-year, $1.25 million contract with the Edmonton Oilers on 17 July 2022.

[10] On 9 November, the Oilers recalled Janmark from their AHL affiliate, the Bakersfield Condors, after Evander Kane suffered an injury.