At MGM, his first credited film was "Maytime", a musical based on the 1917 operetta. In part due to the success of his own children's book "The Land of Green Ginger", he was one of the screenwriters chosen for the job of adapting L. Frank Baum's children's novel "The Wonderful Wizard of Oz" for film. In 11 days, he provided a 43-page adaptation. Changes he introduced to the story are the inclusion of the actors playing the Tin Man, the Scarecrow, and the Lion characters as farmworkers in the sepia tone Kansas sequences as well as changing the color of Dorothy's shoes from silver to ruby. Langley also introduced Miss Almira Gulch, the Wicked Witch's Kansas counterpart. He then wrote a final draft. However, unbeknownst to him, MGM hired Florence Ryerson and Edgar Allan Woolf to do rewrites. But, producer Arthur Freed was displeased with their work and turned the script back over to Langley. Langley disliked their changes and removed many of them. He felt that their version was "so cutesy and oozy that I could have vomited." The final film was released in August 1939. Langley was dismayed by the end result. He said, "I saw it in a cinema on Hollywood Boulevard at noon and I sat and cried like a bloody child." However, he amended his opinion when he saw the film for a second time in England during its 1949 re-release: "I thought, 'It’s not a bad picture. Not a bad picture, you know'."