[1][2] Grün composed chamber music and songs and took work as a conductor in Karlsruhe and Mannheim before joining the Comedy House (Komödienhaus) in Vienna.
[5] His first major work was the 1929 Bohemian Musicians,[2] performed in Vienna in 1930, and he composed music for the soundtrack of the 1932 film Ein Auto und kein Geld.
[8] In the postwar period, Grun continued writing film music for titles such as White Cradle Inn (1947), Balalaika, The Blind Goddess and Brass Monkey (all from 1948).
[10] Grun was credited as a composer alongside John Jerome, which was a collaborative pseudonym for Harold Cornelius Fields, Howard Ellington Barnes and Joseph Dominic Roncoroni.
In addition to his own output, he was also responsible for adapting various musical works, including Bizet's Carmen, Lehár's Count of Luxembourg, Millöcker's Dubarry, and Benatzky's White Horse Inn.