He studied harmony with Thorleif Eken (1900–1955), composition with Klaus Egge and with Niels Viggo Bentzon at The Royal Danish Academy, as well as the piano and double bass at the Oslo Conservatory of Music.
He also participated in the Darmstadt summer school and in the classes conducted by Karlheinz Stockhausen at the Studio für Elektronische Musik in Cologne.
From 1963-73, he was a music critic in Dagbladet, and he was also for many years correspondent for the major German magazine Melos.
[4] Rolf Wallin, Jon Mostad, Lasse Thoresen, Terje Bjørklund and Synne Skouen are among his students.
It later assimilated twelve-tone and aleatoric influences, creating what Mortensen termed a "neo-serial" style.