During the 2009–10 season, Janmark played his first full season with AIK in Stockholm in Sweden's top division after joining the club mid-season from the lower division club SDE HF the previous year. He played for AIK's U20 and U18 teams – appearing in 17 games, including four playoff games, with the U20 team and playing in 34 U18 contests, including one playoff game. Janmark recorded four goals and eight assists in U20 play and had 13 goals with 22 assists for AIK's U18 team. During the 2010-11 season, Janmark skated for AIK Stockholm's U20 team, playing in 40 Super Elit league games. He recorded 11 goals and 17 assists. AIK finished sixth in the Super Elit league's North Division. During the 2011–12 season, Janmark made his pro debut in Sweden's Elitserien, appearing in 18 games for AIK Stockholm. He had no points in limited ice time in Elitserien play. He appeared in three playoff games with AIK with no points or penalties. Janmark also was the team captain for the club's U20 team. In 40 games for the club's U20 team he was the team's leading scorer with 23 goals and 38 assists. AIK finished fourth in the U20 standings but lost to Färjestads in a first round playoff series. In his second year of draft eligibility, Janmark was ranked 98th amongst European skaters in Central Scouting's final rankings but was not selected in the 2012 NHL Entry Draft. During the 2012–13 season, Janmark had an impressive first full season in the Elitserien, finishing as AIK's second-leading scorer behind Broc Little. In 55 games Janmark recorded 14 goals and 17 assists. AIK missed the Elitserien playoffs, finishing six points behind eighth-place Brynäs for the final spot. Following this season, Janmark was drafted by the Detroit Red Wings. 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. He made his American Hockey League debut on 18 April in a game against the Lake Erie Monsters. For the following 2014–15 season, Janmark opted to remain in Sweden, signing a one-year contract with Frölunda HC on 3 June 2014. On 1 March 2015, the Red Wings traded Janmark, Mattias Bäckman and a third round draft pick to the Dallas Stars in exchange for Erik Cole and a second round draft pick. 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. He was immediately assigned to the Stars' minor league affiliate, the Texas Stars for their playoff run. 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. After a successful rookie year, he would miss the entire season in 2016–17 due to a knee injury. On 6 June 2019, the Stars re-signed Janmark to a one-year, $2.3 million contract extension.
