Conductor and composer of multiple film scores, including "A Night in Casablanca" from 1936. He formed the Janssen Symphony in 1940.
He studied piano with one of Franz Liszt's students. He received a Juilliard Fellowship. He created many new compositions, including "Louisiana Suite," while he studied in Rome.
His first official film score, "The General Died at Dawn" from 1936, received an Academy Award nomination. He also scored "Blockade" from 1938, "The House Across the Bay" from 1940, and "Uncle Vanya from 1957.
His father was the owner of the New York restaurant Hof Brau Haus. He married Ann Harding with whom he had a child named Grace Kaye. His second wife, Christina Heintzmann, had three children with him: Werner Janssen Jr., Alice Janssen Krelle, and Jennifer Janssen.
He and Jerome Kern both composed music for Tin Pan Alley.