He played C. W. Moss in Bonnie and Clyde, for which he received Academy Award and Golden Globe Award nominations for Best Supporting Actor and won a British Academy Film Award for Most Promising Newcomer to Leading Film Roles.
He is noted for his short stature, which had him playing child roles well into his twenties (including on Star Trek, where he played one of the inhabitants of the planet of children in the episode "Miri") and resulted in a recurring role as the diminutive trans-dimensional imp Mr. Mxyzptlk in the Superboy television series.