Born in Louisiana but raised in Chicago, Hartman began singing and playing the piano by the age of eight. He attended DuSable High School studying music under Walter Dyett before receiving a scholarship to Chicago Musical College. He sang as an Army private during World War II, but his first professional work came in September 1946 when he won a singing contest awarding him a one-week engagement with Earl Hines. Seeing potential in the singer, Hines hired him for the next year. Although Hartman's first recordings were with Marl Young in February 1947, it was the collaboration with Hines that provided notable exposure. After the Hines orchestra broke up, Dizzy Gillespie invited Hartman to join his big band in 1948 during an eight-week tour in California. Dropped from the band about one year later, Hartman worked for a short time with pianist Erroll Garner before going solo by early 1950.