[2] 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.

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.

[7] 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.

[10] 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.

[12] 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.

[14] 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.

[16] 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 coach Jeremy Colliton.

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.

[19] On 27 January 2020, Martinsen returned to Europe and agreed to join Swiss club, EV Zug of the National League (NL), for the remainder of the season.

[21] In the third season of his stint with Lillehammer in 2022–23, Martinsen posted 8 points through 15 games before leaving the club for fellow Norwegian outfit, Vålerenga, on 2 January 2023.