He was a member of Roy Hellvin Trio, was in the backing band at Kongsberg Jazz Festival in 1967 and 1968, was elected Best Bassist by Jazznytt in 1967, and started as bass player in the Jan Garbarek Quartet (1967–1973), including Terje Rypdal and Jon Christensen.

[3] After completing his technical education in 1968, he became a professional musician and collaborated with Karin Krog, George Russell, and Don Cherry (Berlin 1968), and with visiting American musicians Phil Woods, Dexter Gordon, Bill Frisell, Hampton Hawes, Johnny Griffin, Sonny Rollins, Sheila Jordan, and Chick Corea.

[1] In the early 1970s, Andersen collaborated with Norwegian musicians Magni Wentzel, Jon Eberson, Ketil Bjørnstad, and Terje Rypdal, before leaving for an eventful visit to the U.S. in the winter of 1973–1974, and has since 1974 led his own bands, at first a quartet (1974–79).

[3] In January 2009, he was named "Musicien Europeen 2008" by the French Academie du Jazz,[4] In 2010, Andersen received the Ella Award at the Oslo Jazzfestival.

[1] In 2022, he recorded a trio album with the American drummer Bob Moses and Slovenian guitarist Samo Salamon entitled Pure and Simple.