Marcus Monsen contested his first FIS races in November 2010 after reaching the age limit , in which, however, he could not classify in the front field. In January 2012 he took part in the Winter Youth Olympic Games in Innsbruck , where he won the silver medal in the team competition. At the beginning of winter 2012/2013 Monsen was able to improve significantly, especially in the technical disciplines of slalom and giant slalom, and won several FIS races in January 2013. In February 2013 he became Norwegian junior champion in giant slalom. Monsen has been regularly competing in the European Cup since the 2013/14 season . After finishing sixth in the giant slalom in Les Menuires on January 27, 2014, he was among the top ten in this racing series for the first time. On February 2, 2014, he contested his first World Cup race in the giant slalom in St. Moritz . There he went straight to 26th with starting number 65 and thus won his first World Cup points.
Monsen did not manage to establish himself permanently in the World Cup. His best result was a 17th place in the giant slalom in Kranjska Gora on March 4, 2017. For the 2019/20 season, he lost his place in the Norwegian national team and then trained with the private team Global Racing . There were no countable results, which is why he announced his resignation on August 10, 2020 with immediate effect.