Gruenberg played both solo concerts and in ensembles from the beginning, and in his early twenties he went to study in Europe with Ferruccio Busoni at the Vienna Conservatory.

In 1919, Gruenberg wrote The Hill of Dreams for orchestra, which gained him the highly acclaimed Flagler Prize and enabled him to devote himself more completely to composition.

Then, on February 4, 1923, Gruenberg conducted the American premiere of Pierrot Lunaire by Arnold Schoenberg at the Klaw Theatrein another concert organised by the ICG.

In 1937, he moved with his family to Beverly Hills, California, where fellow League members Arnold Schoenberg and Igor Stravinsky also now lived (though they never spoke).

Gruenberg worked on the musical scoring for John Ford's masterpiece Stagecoach (1939), incorporating folk songs; the four other composers who worked with him are named on the Academy Award Stagecoach won for Best Music Scoring in that legendary year – against such tough competition as Dimitri Tiomkin, Erich Korngold and Aaron Copland – but Gruenberg, inexplicably, is not on the list of nominees.

[5] In 1942, Gruenberg was again nominated for his next film, along with Columbia's head of music, Morris Stoloff, for Best Dramatic Score for Commandos Strike at Dawn (1942), directed by John Farrow and starring Paul Muni in a story of a secret Allied attack on the Nazi-occupied Norwegian coast.

Originally, Stoloff had convinced his boss, Harry Cohn, to hire Stravinsky for this job, since the Russian genius happened to be sitting out the war in Los Angeles.

When the prolific composer finished his score before a single frame of film had been shot; Stoloff ruefully paid Stravinsky and gave the work back.

a legendary performance of the master at the height of his powers, and is frequently re-issued on LP paired with Heifetz's 1937 recording of the Prokofiev Violin Concerto.

Gruenberg's last Hollywood score was for Mickey Rooney and Peter Lorre in the film noir Quicksand (1950), directed by veteran Irving Pichel, and considered by many[who?]

Again, the orchestral opening under the credits has a powerful use of brass, and as Rooney descends into his life of petty crime, a sinewy jazz score emerges.

[8] Besides other works, he wrote five symphonies, four full length operas (Volpone, Jack and the Beanstalk, Antony and Cleopatra and The Dumb Wife) and the lengthy oratorio A Song of Faith.