Having attended the University of Illinois, Parks played in stock companies for several years before signing a Hollywood contract in 1941. He made several forgettable films untl he won the role of Al Jolson in the 1946 biographical film The Jolson Story, for which he was nominated for an Academy Award for Best Actor.
Following his Oscar nomination, he made a few more movies that did little box office, until he appeared in the sequel, Jolson Sings Again in 1949, which was another hit.
Following his admission before the House Un-American Activities Committee, Columbia Pictures dropped him. He made only two more films before retiring permanently.