born in sandefjord, monsen graduated from teacher's college in 1977 and worked at fevang school from 1978 to 1997. he was a member of the executive committee of sandefjord city council from 1987 to 1997, serving as deputy mayor in 1990–1994 and 1995–1997. he chaired the local party chapter from 1994 to 1997. in 2007 he was again elected to the city council.