Despite The Doobie Brothers' success, Hartman grew weary of the internal tensions and illness that tore at the group, and left in 1979.
He told Rolling Stone in 2020 about the reason for his exit. He felt "everything was falling apart" within the band, which was inducted into the Rock and Roll Hall of Fame the same year and sold more than 40 million records globally.
Hartman tried to become a police officer after leaving the band, graduating from a reserve police academy.
He was rejected by 20 police departments across California due to his famous past and history with drugs.
Hartman told the New York Times in the 1990s about how his previous marijuana use became a "major" liability to his attempt at starting a new career in law enforcement, adding: "These guys still think I'm a credibility problem because of what I used to do. I've picked myself up from the sewer."
Hartman then rejoined The Doobie Brothers at a reunion show for Vietnam veterans in 1987 and their comeback album Cycles in 1989.
He said about calming down as he aged along with the band, best-known for hits including What A Fool Believes: "The road treats us the same, we just don't treat it the same. We're not trashing hotel rooms anymore and we're not having door wars with rent-a-cars, burning up stages and things of that nature."
Born in 1950 in Falls Church, Virginia, Hartman became a musician and part of the booming 1970s scene in California.
He formed The Doobie Brothers with frontman Tom Johnston, now 74 and still the face of the group.
