Born in Waukegan, Illinois of German descent, he started as a child actor in the 1930s and appeared opposite Conrad Nagel and Leslie Howard, amongst others. His first fame came in playing the role of Beau Wilkes (Ashley and Melanie's son) in Gone with the Wind (1939). He later went on to appear in Juarez (1939) opposite Bette Davis, as the adoptive son of John Wayne in Red River, and in A Streetcar Named Desire (1951), in which he was reunited with Vivien Leigh a dozen years after they first worked together in Gone with the Wind. In A Streetcar Named Desire, Kuhn played a Sailor who directs Blanche to the correct streetcar which will take her to her sister's neighborhood at the beginning of the film. He is the only actor to share screen time with Leigh in each of her Oscar-winning performances.