[2] Carroll developed into an excellent singer and tap dancer at the Fanchon and Marco Dancing School in Hollywood,[3] and became an accomplished child actress.

Her big break came the 1940 film, Primrose Path, as Ginger Rogers's younger sister, for which she won a Critics Award.

The same year she became the first child star to be summoned from Hollywood in order to appear in the leading role in a Broadway musical, Panama Hattie, which ran from October 30, 1940, to January 3, 1942.

RKO starred Carroll in the leading role with Ruth Warrick in two zany comedy vehicles, Obliging Young Lady (1941) and Petticoat Larceny (1943).

Two of her best-remembered pictures came from this period: Meet Me in St. Louis (1944) as Judy Garland and Margaret O'Brien's sister, and The Bells of St. Mary's (1945), in which she played "the sensitive child of separated parents.