Day began her career as a big band singer in 1939. Her popularity began to rise after her first hit recording, “Sentimental Journey”, in 1945. After leaving Les Brown & His Band of Renown to try a solo career, she started her long-lasting partnership with Columbia Records, which would remain her only recording label. The contract lasted from 1947 to 1967, and included more than 650 recordings, making Day one of the most popular and acclaimed singers of the 20th century. In 1948, after being persuaded by Sammy Cahn,Jule Styne and her agent at the time, Al Levy, she auditioned for Michael Curtiz, which led to her being cast in the female lead role in Romance on the High Seas.
Over the course of her career, Day appeared in 39 films.