He finished fifth at the 1986 European Championships, fifth at the 1987 World Indoor Championships, tenth at the 1990 European Indoor Championships, sixth at the 1990 European Championships and fifth at the 1991 World Indoor Championships. He became Norwegian champion in 1989 and 1991. Later in 1991 he won the bronze medal at the World Championships in Tokyo with a throw of 20.75 metres, but as the silver medalist Georg Andersen was later disqualified from the competition because of using a banned substance, Nilsen was promoted to silver medal position. Nilsen's last international competition was the 1992 European Indoor Championships, where he finished seventh.