He attended the University of Music and Performing Arts and worked as a playwright and director in the German theatre.
[1] Mittler also spent time at the American company Paramount's French subsidiary based at the Joinville Studios in Paris.
Following the Nazi rise to power in 1933, Mittler spent many years in exile in several countries, including Britain and France, before settling in the United States during the Second World War.
Mittler's career as a director had all but ended in the mid-1930s, after making the Stanley Lupino musical comedy Cheer Up (1936), but he worked occasionally as a screenwriter.
Mittler wrote the original story of the MGM pro-Soviet film Song of Russia (1944) which was later investigated by the House Un-American Activities Committee for its alleged communist sympathies.