Magnus Rosén started to play bass when he was 15 years old. When he had played bass for three years he went on his first tour with the band Kung Sune. After the tour he moved to Los Angeles, there he played in many bands, like Billionaires Boys Club where future Hammerfall-bandmate Anders Johansson played drums. He became a member of the Swedish heavy metal band Hammerfall in 1997. Magnus is also a jazz bassist, having recorded several albums of solo material. He occasionally does solo tours in South America, where he donates the profits to charity. On March 7, 2007, Magnus left Hammerfall to focus on his own projects.
Rosén has recently become involved in former Black Sabbath vocalist Tony Martin's live band, Tony Martin's Headless Cross. The band played their debut show on July 27, 2012, at The Asylum in Birmingham, UK. The show was filmed for a live DVD, and is intended as a warm-up for a full tour in 2013.
In 2015, Magnus was announced as the new bassist for Brazilian heavy metal band Shadowside.