After having met Edvard Grieg shortly before his death (at the age of 6) he turned to music and studied piano with Nils Larsen and Fridtjof Backer-Grøndahl, composition with Otto Winter-Hjelm and Gustav Lange. His brother, violinist Finn Grüner-Hegge, joined him for his debut concert as a composer (1917). Additionally he turned to conducting and studied in Berlin, Vienna and Basel, amongst others with Camillo Hildebrand and Felix Weingartner. He began a successful conducting career in Norway and in 1931 he became bandmaster (“kapellmester”) for the Oslo Philharmonic Orchestra, together with Olav Kielland. He had to resign two years later after a political and aesthetical controversy with Kielland which aroused a strong public debate. In 1934 he was appointed “kapellmester” at Oslo’s Nationaltheatret and was invited to conduct the Berlin Philharmonic Orchestra, with very positive reviews, so that he continued to perform internationally.
He was first married (1929 to 1944) to the Jewish-born Sigrid Elena Feinsilber (born 3 March 1906). After she had been arrested due to Nazi-persecution, imprisoned in Bredtveit 26 November 1942 and released 3 December 1942 she escaped to Sweden and became a Swedish citizen. In 1944 he married Karen Welle (1914-2006).
Due to a sustantial lack of primary sources and inaccessible personal papers his role during the years of the German occupation still is rather opaque. It seems that he managed to meander ambigously between continuing his artistic career while at least trying to avoid open collaboration.