He has written many critical books, including What is Theatre?, The Life of the Drama, Theatre of War, Brecht Commentaries, and Thinking about the Playwright. His most-produced play, Are You Now Or Have You Ever Been, about the Un-American Activities committee hearings, was published in 1972. Another play, Lord Alfred's Lover, deals with Oscar Wilde.
Bentley met Bertolt Brecht at UCLA as a young man and is considered one of the pre-eminent experts on Brecht, much of whose work he has translated. He edited the Grove Press issue of Brecht's work, and recorded two albums for Folkways of Brecht / Eisler songs
In the 1960s, Bentley came out of the closet and declared his homosexuality. He has stated his being gay as an influence on his theater-work, especially his play "Lord Alfred's Lover".