 His first credited film score was for "The General Died at Dawn" (1936), which was nominated for an Academy Award, the first of six Janssen scored films to be nominated.In 1939, he resigned his position with the Baltimore Symphony Orchestra to work with film producer Walter Wanger.He composed several other film scores including "Blockade" (1938), "Winter Carnival" (1939), "Eternally Yours" (1939), "Slightly Honorable" (1940), "The House Across the Bay" (1940), "Guest in the House" (1944), "The Southerner" (1945), "Captain Kidd" (1945), "A Night in Casablanca" (1946), "Ruthless" (1948), and "Uncle Vanya" (1957), starring and co-directed by Franchot Tone.