Fossen grew up in Engerdal and Vågå. He came to Trondheim in 1962 where he played as goalkeeper in Rosenborg. Until 1969 Fossen played 55 matches in the top division. In 1970 a new goalkeeper, Geir Karlsen, arrived at Rosenborg, and Fossen was benched. He then became assistant coach under George Curtis, and in 1972 head coach together with Nils Arne Eggen coaching Rosenborg to the double, winning both the league and the cup titles. Fossen continued without Eggen 1973–1974, and the team became second both in the league and in the cup 1973. Fossen coached Start 1975–1977[1] to bronze medal in the league 1975. Besides this Fossen also coached Norway national team for juniors 1972–1976.
^""Trene gjennom tidene"". Archived from the original on 18 July 2012. Retrieved 14 September 2012.{{cite web}}: CS1 maint: bot: original URL status unknown (link) (in Norwegian). Strømsgodset IF. Retrieved 20 March 2013.