Martinsen first played as a youth in his native Norway with Rosenborg IHK before joining GET-ligaen first team club, Lillehammer IK in 2006. As a 17-year-old, Martinsen made his first team professional debut with Lillehammer in the 2007–08 season, featuring in 43 games with a promising 20 points. With good size and physical play, Martinsen was hopeful to be selected in the Canadian Hockey League import draft but was passed over. On 7 July 2008, he opted to remain with Lillehammer to continue his development on a year contract. After his second full season with Lillehammer, establishing himself amongst the scoring lines with 23 points in 43 games, Martinsen signed a contract with Swedish HockeyAllsvenskan outfit, Leksands IF on 9 April 2009. During the 2009–10 season, Martinsen struggled to cement his position within Leksands. Used sparingly through 22 games, Martinsen agreed to leave Sweden to return to Lillehammer on loan for the remainder of the season on 15 December 2009. As a welcome addition to the Lillehammer club, Martinsen immediately contributed to finish the year with 10 points in 14 games. On 29 May 2010, Martinsen was signed to a multi-year contract to remain with Lillehammer. Over the next two seasons, Martinsen established himself amongst the club's scoring leaders on a point per game average. In order to progress his career on 25 May 2012, he agreed to his second European venture in signing with German club, Düsseldorfer EG in the Deutsche Eishockey Liga. In his rookie DEL season in 2012–13, Martinsen secured a checking line role on DEG, contributing with 22 points in 52 games as a 22-year-old. After signing for a third season with Düsseldorfer EG, Martinsen recorded a break-out season in 2014–15, using his size and touch to finish third on the team in scoring with 18 goals and 41 points in 55 games. He made his post-season debut with DEG appearing in 12 games with 5 points upon suffering a semi-final loss. With his breakout performance garnering NHL interest, Martinsen was announced to have signed a one-year, two-way entry-level contract with the Colorado Avalanche on 15 May 2015. After attending the Avalanche September training camp, Martinsen was reassigned to begin the 2015–16 season with American Hockey League affiliate, the San Antonio Rampage. After 9 games with the Rampage, Martinsen was recalled to provide a physical presence to Colorado on 9 November 2015. Martinsen made his NHL debut the following day in a 4-0 victory over the Philadelphia Flyers. He became the third Norwegian player to appear with the Avalanche and just the 8th to reach the NHL. In his 6th game with the Avalanche, Martinsen recorded his first NHL goal in a 7-3 defeat to the Washington Capitals on 21 November 2015. Martinsen remained primarily with Colorado for the remainder of the season, appearing in 55 games for 11 points and finished the year leading all NHL rookies in hits-per-game. On 24 June 2016, Martinsen agreed as a restricted free agent to sign a one-year, one-way contract extension with the Avalanche. In the 2016–17 season, Martinsen made the opening night roster, solidifying his role as a physical checking forward on the fourth-line. He appeared in his 100th career NHL game in a 4-1 defeat to the Pittsburgh Penguins on 9 February 2017. With the Avalanche in the midst of their worst season in franchise history in Colorado, Martinsen matched his previous years tally of 55 games before he was dealt at the NHL trade deadline to the Montreal Canadiens in exchange for Sven Andrighetto on 1 March 2017. He became the first Norwegian to be involved in a NHL trade at the deadline. Martinsen played out the season with the Canadiens, going scoreless in 9 regular season games and 2 playoff games. On 6 June 2017, Martinsen was re-signed to a one-year, one-way deal with the Canadiens. After participating in Montreal's training camp and pre-season, Martinsen failed to make the club and was reassigned to AHL inaugural affiliate, the Laval Rocket, on 3 October 2017. The following day, on the opening of the 2017–18 season, Martinsen was traded by the Canadiens to the Chicago Blackhawks in exchange for Kyle Baun. He would continue in the AHL with the Rockford IceHogs. On 7 June 2018, Martinsen agreed to a one-year extension to remain with the Blackhawks. Making the Blackhawks opening night roster for the 2018–19 season, Martinsen appeared in 24 games registering 1 goal and 4 points, before he was sent down to the AHL after his role diminished under new head coachJeremy Colliton. He played the remainder of the campaign with Rockford, compiling 11 points in 38 games. On 5 July 2019, Martinsen signed as a free agent to a one-year, two-way contract with the Anaheim Ducks. After attending the Ducks 2019 training camp, Martinsen was assigned to begin the 2019–20 season, with AHL affiliate, the San Diego Gulls. He was scoreless in a lone appearance with the Gulls before he was traded by the Ducks along with a 2021 seventh-round draft selection to the Pittsburgh Penguins in exchange for Erik Gudbranson on 26 October 2019. He was immediately assigned to continue in the AHL with affiliate, the Wilkes-Barre/Scranton Penguins. In joining Wilkes-Barre, Martinsen struggled to find his role, producing just three points through 24 games. On 24 January 2020, Martinsen was placed on unconditional waivers by the Penguins, in order to complete a mutual termination of his contract. On 27 January 2020, Martinsen returned to Europe and agreed to join Swiss club, EV Zug of the NL, for the remainder of the season.
International play
Martinsen was first selected to compete at the international stage as a junior for Norway at the 2007 World Under-18 Championships. Finishing with a Division 1 Bronze medal, Martinsen was a fixture in the Norwegian junior team throughout his junior eligibility. Martinsen made his Senior international debut for Norway as a 20-year-old at the 2010 IIHF World Championships. He has appeared in each World Championships since, marking 7 consecutive Championships as of 2016.
