At 17 Hart obtained a job as office boy for the theatrical producer Augustus Pitou. He wrote his first play at 18, but it was a flop. He then worked as director of amateur theatre groups, spending his summers as entertainment director of vacation resorts in the Catskills, known in the theatrical world as “the borscht circuit.” In 1929 he wrote the first draft of Once in a Lifetime, a satire on Hollywood that became a hit the following year, after its exuberant humour had been tempered by the sardonic skill of George S. Kaufman. Hart then wrote books for musicals for Irving Berlin and Cole Porter; but until 1941 he continued to work with Kaufman, a collaboration that produced such popular comedies as You Can't Take It with You (1936) and The Man Who Came to Dinner (1939). His success continued with his musical play Lady in the Dark, which he himself directed in 1941. Among other plays he directed was the long-running My Fair Lady (1956). In 1959 he published Act One, the story of his theatrical apprenticeship.