He was born in Kristiania as a son of Ludvig Monsen (1854–1942) and Josefine Aurora Marcelie Dehn (1852–1942).

[1] Monsen edited the newspaper Demokraten from 1913 to 1916 and was a member of Hamar city council from 1907 to 1945, serving as mayor in 1916–1919.

He was elected to the Parliament of Norway from the Market towns of Hedmark and Oppland counties in 1922, and was re-elected on six occasions.

Unusually for a Minister of Defence, Monsen was an antimilitarist and wrote three anti-militarist pamphlets (Sannheten om militærvesenet, Avvæbning eller militarisme and Militært vanvidd eller civil fornuft).

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