Born in Harrisburg, Pennsylvania, Hartman joined his first band, The Legends, at the age of 13, with his brother Dave. Hartman played keyboards and wrote much of the band's music, but although several records were released they were not hits. He joined the Edgar Winter Group and played on three of their albums before launching a solo career in 1976. He achieved two solo hit singles "Instant Replay" (1978) and "Relight My Fire" (1979) during the disco era.
His biggest solo success was the top 10 single "I Can Dream About You" from the Streets of Fire soundtrack in 1984. During the next decade he worked as a songwriter and producer and collaborated with such artists as Tina Turner, Joe Cocker, Bonnie Tyler, Paul Young, James Brown, Nona Hendryx and Steve Winwood.
He died in Westport, Connecticut from a brain tumor caused by AIDS.
At the time of his death, Hartman's music was enjoying a phase of revival and a cover version of his song "Relight My Fire" became a British number one hit for Take That and Lulu.