Joan Felt was born on January 18, 1931, in Elizabeth, New Jersey. Her mother was quite a famous piano player in the 1930s. Six-year-old Joan made her film debut in Walking Down Broadway (1938). She played the role of Sunny, and changed her name from Felt to Carroll. A role in Two Sisters (1938) followed, and the next year she had supporting roles in Barricade (1939) and Tower of London (1939). It wasn't until 1940 when Joan had her breakthrough. She had important parts in Anne of Windy Poplars (1940) and especially Primrose Path (1940), as Ginger Rogers' younger sister. In 1941, she won her first lead role in Obliging Young Lady (1942) as Bridget Potter, a young girl stuck in the middle of her parents' divorce case. The film costarred Ruth Warrick. In 1942, she was the first child star from Hollywood to appear in a Broadway play.This play, "Panama Hattie", garnered Carroll national fame, and she was featured in many magazine articles and newspapers. In 1943, she won her second lead role in Petticoat Larceny (1943), in which she played Joan Mitchell, a radio star who goes undercover to get a better feel of her roles. That film reunited Joan with Warrick. In 1944, she played Agnes, the middle sister between Judy Garland and Margaret O'Brien in Meet Me in St. Louis (1944). In 1945, she had an important supporting role in The Bells of St. Mary's (1945), which starred Bing Crosby and Ingrid Bergman. That same year she appeared in Tomorrow, the World! (1944), after which she retired.