He was an editor in Arbeiderbladet from 1952 to 1964 and 1968 to 1970, director of the International Press Institute from 1964 to 1968 and editor-in-chief of the Norwegian News Agency from 1970 to 1980.
He was born in Hamar as a son of politician Fredrik Monsen (1878–1954) and teacher Aasta Ingerø Hansen (1884–1962).
[1] In 1940 the occupation of Norway by Nazi Germany had begun, and Monsen was a member of the Norwegian resistance movement.
[1] In the Labour Party, to which the newspaper Arbeiderbladet belonged, Monsen was known as a "hardliner" in foreign and security policy.
His brother-in-law Halvard Lange served as Minister of Foreign Affairs from 1945 to 1965, and was a proponent for a pro-American policy in the Cold War.