He studied electroacoustic music in Utrecht, and musique concrète and spectral music in Paris.
He has been professor of composition at the Norwegian Academy of Music since 1988. Thoresen has received many commissions for works which have been performed across Scandinavia, including commissions from all the major Norwegian Philharmonic Orchestras and the French National Radio. An important source of inspiration for Thoresen has been the Bahá"í religion whose texts he has set in many works.
His oratorio was performed at the inauguration of the 19 terraces of the Bahá"í holy place on Mount Carmel, by forces from Canada, Austria, Israel and Transylvania.
Member of Norsk Komponistforening (member board 1975-1979, Work of Year prize 1980, 1993, 1995).
Married Britt Strandlie Thoresen.