He was active in the resistance during the German occupation of Norway. While being a teaching student he published the clandestine bulletin Fritt fram (Norwegian for "Freely forward"). Knutsen, along with a group of colleagues, was arrested in 1944 and Fritt fram ceased publication.
During the 1950s he stayed in Moscow, and worked as a newsreader for the Norwegian-language broadcasts of Radio Moscow.
On 5 March 1953 he was the first to read out the news of the death of Joseph Stalin to a Norwegian audience. Knutsen replaced Larsen as party chairman in 1975, and pulled the party out of SV. After the rupture with SV the marginalization of NKP continued.
He headed the NKP candidate slate in Akershus in the 1977 legislative election, but was not elected, as the party obtained a meagre 0.4% of the votes nationwide. He stepped down from the NKP chairmanship in 1982.
Knutsen left NKP in September 1990, after five decades of membership in the party.
He died in 2001.
Knutsen headed the orthodox group inside NKP, which resisted the moves by the party chairman Reidar T. Larsen to merge the party into the Socialist Electoral League (SV).
Knutsen had been a deputy member of the Parliament of Norway during the term 1973–1977, and met during 2 days of parliamentary session. Knutsen was also a member of the executive committee of Skedsmo municipal council from 1971 to 1975.