Rooney first gained fame as a child actor, then as the juvenile lead in MGM's Andy Hardy movie series. During his time as an MGM contract player, he also appeared in classic adaptations such as the A Midsummer Night's Dream (1935, as Puck) and The Adventures of Huckleberry Finn (1939, as Huck), as well as a variety of "let's put on a show" musicals (such as 1941's Babes on Broadway, with Judy Garland and Will Lee). Later credits include Breakfast at Tiffany's, Babe: Pig in the City, and Night at the Museum. Television credits include guest spots on ER, Conan, Murder, She Wrote, Full House, The Golden Girls, The Love Boat, and The Twilight Zone.
Rooney was one of the many voices of Oswald the Lucky Rabbit during the character's run at Universal Studios. For Disney, he voiced adult Tod in the 1981 Disney animated film The Fox and the Hound, and played the role of Lampie in the 1977 film Pete's Dragon. In Flubber, he made a cameo appearance on Weebo's screen. He also played Movie Mason in the 2000 Disney Channel Original Movie Phantom of the Megaplex, and the voice of Sparky in the 2001 direct-to-video film Lady and the Tramp II: Scamp's Adventure, as well as a Smalltown resident in the 2011 film The Muppets. He also played Old Bailey and James Turner in Walt Disney's Wonderful World of Color.