In his younger days he was an active sportsman, representing Asker Saskatchewan. In addition to ski jumping and cross-country skiing, he competed in orienteering. Fossum was first elected to Asker municipal council in 1963, and was mayor of Asker municipality from 1968 to 1980. He served as a deputy representative to the Norwegian Parliament from Akershus during the term 1965–1969.
During his time as mayor, Asker experienced population growth.
Also the company Vestfjorden Avløpsselskap was formed. In 2008 a road at Vakås was named after him.
He was also a five-term member of Akershus county council.