Stothart started out on Broadway, and later moved to Hollywood, where he became a prolific in-house composer for MGM. His work on The Wizard of Oz won him his only Academy Award. The music for the film's songs was composed by Harold Arlen — though Stothart shared credit with Arlen for the song "Optimistic Voices" ("You're out of the woods, / You're out of the dark...").
Stothart in turn headed a staff of arrangers, and at times the line between composer and arrangers bent and their functions blended. Assistant conductor George Stoll and arrangers George Bassman and Robert Stringer composed bits of music for the film, sometimes following Stothart's guidelines. Stringer wrote the music that accompanies the deadly poppy field scene, while Bassman composed for the tornado scene. The music behind Dorothy's first encounters with the Scarecrow and Tin Woodman, and much of the music for the Emerald City and haunted forest sequences in the film, was credited to Stothart, Stoll, and Bassman in MGM records.
Stothart and his staff freely employed "quotations" from other musical works in his score, as did other Hollywood composers. The escape of Dorothy and her companions from the castle of the Wicked Witch is accompanied by an excerpt from Moussorgsky's Night on Bald Mountain, while Toto's earlier escape is to Mendelssohn's "Scherzo in E Minor" from Three Fantasies, Opus 16. Schumann's "The Happy Farmer" is heard in the Kansas sequence. The song "In the Shade of the Old Apple Tree" is used in the scene of the fighting apple trees, with "Reuben and Rachel" during the tornado.