 Christensen graduated in art history from the University of Oslo in 1955.He opened his own gallery in Oslo in 1961, Galleri Haaken, which he was running for about forty years.Among his works is his thesis about Gustave Courbet, two books about Olivier Debré, essays on Serge Poliakoff, Horst Janssen and Alfred Manessier, and the memoir book "En gallerists erindringer" from 1985.