Nilsen made his debut as a student at Nationaltheatret in 1959 and graduated from the Norwegian National Academy of Theatre in the same year.
He has since worked with Trøndelag Teater in 1961-68, Det Norske Teatret in 1968-73, Fjernsynsteatret in 1974-77, and Nationaltheatret since 1977.
At the Trøndelag Teater, he made his debut in 1966 as the narrator of the piece Heat wave by Ted Williams.
At Det Norske Teater he took part in translated productions such as Myrfolket, Johann uten land, Montserrat and not least, playing van Gogh in Postmannen fra Arles.
In the 2000s, he has acted in works including The Master Builder (2003), Maria Stuart (2004) and Til Damaskus (2006), all with Nationaltheatret.