With longtime collaborator
He went to Columbia University and Columbia Law School, but dropped out of law school to embrace theatre full-time. His first musical was Always You from 1921.
His first collaboration with Rodgers was a musical adaptation of the play Green Grow the Lilacs. Their adaptation would be called Oklahoma!, opening on Broadway in 1943. They went on to write Carousel, The King and I, South Pacific, The Sound of Music, and many other classics that developed the idea of the songs advancing and establishing the characters in the play.
He married Myra Finn on August 22, 1917; they divorced in 1929. He married Dorothy Jacobson on May 13, 1929. He had two children by Myra and one child, future theatre director and producer James Hammerstein, by Dorothy.
He also collaborated with Jerome Kern, for which Show Boat from 1927 was their biggest hit.