Thoresen was born in Oslo on October 18, 1949. He attended the Norwegian Academy of Music, studying with Finn Mortenson and graduating in 1972. He went on for further studies in Utrecht in the Netherlands, where he studied electroacoustic music with Werner Kaegi at the Institute of Sonology and then moved to Paris, where he immersed himself in musique concrète and spectral music. These genres have influenced his mature music, but the influence is indirect; his works are generally for conventional instruments. Among his early works were the song The Garden (1976), With an Open Hand or a Clenched Fist?, recorded by flutist Maiken Mathisen Schau on his 2016 album Sea of Names, and the piano trio Bird of the Heart (1982). In 1988, Thoresen joined the faculty of the Norwegian Academy of Music, where he remained as of the early 2020s.
Thoresen's works have been widely performed, with all the major Norwegian orchestras as well as French National Radio commissioning new orchestral works from him. One major impetus behind his compositional activity has been his adherence to the Baha'i faith; his 2000 oratorio Terraces of Light was performed on the 19 terraces of a Baha'i holy site on Mount Carmel, Israel, by forces from four countries. He has also written chamber music and keyboard music. Several of Thoresen's works place aspects of Norwegian folk music in modern contexts, including To the Brother Peoples, a double concerto for hardingfele and nyckelharpa (2005). By the early 2020s, more than 25 of Thoresen's works had been recorded, including Lyden av Arktis ("Sound of the Arctic"), commissioned and recorded by Norway's Arctic Philharmonic Orchestra in 2022. ~ James Manheim, Rovi