Born in New York, Byrnes made his professional debut with Joe E Brown's Circus Show. He arrived in L.A. on September 30, 1955, the day James Dean died in an automobile accident. Within two years, he had made his screen debut alongside Anthony Perkins in "Fear Strikes Out" (1957) and went on to supporting roles in such efforts as "Reform School Girl" (also 1957), "Darby's Rangers" (1958), as an uncharacteristically nasty army lieutenant, and "Marjorie Morningstar" (1958).
Byrnes was cast as a hired killer in the B-movie "Girl on the Run" (1958) which served as the pilot for "77 Sunset Strip". During filming, the director asked the actor to create a piece of business indicating the character's coolness; Byrnes created what was to become one of his signature moves, whipping out a comb to run through his pompadour. When the series was picked up by ABC, Byrnes was hired to play a carhop who wants to be a detective. Serving as the show's comic relief, the character became noted for his trademark slang (e.g., "the ginchiest" meaning cool or hip, "keep the eyeballs rolling" meaning be on the lookout) and his attention to his looks. Byrnes solidified his teen idol status by recording the 1959 hit single "Kookie, Kookie, Lend Me Your Comb" with Connie Stevens.
Unhappy that the show's producers would not allow him to pursue movie opportunities and stuck a paltry $500 per week salary despite his popularity, Byrnes walked off the show (and was briefly replaced by Troy Donahue). He returned to the show, with his character promoted to partner in the detective agency. When ratings began to decline in 1963, the show was overhauled and the cast (save for Efrem Zimbalist Jr) was replaced. Byrnes soon found that his own popularity was waning. He landed roles in undistinguished motion pictures like "Beach Ball" (1965) and remained active making guest appearances on TV series. Off-screen problems, however, also took their toll. In his 1996 autobiography, Byrnes detailed his addictions to heroin and alcohol. Despite occasional feature work, it was not until 1978 that he landed a role in a hit film; Randal Kleiser added him to the nostalgia-ladened cast of "Grease" in the role of a lecherous disc jockey. Despite the success of the film, Byrnes was unable to translate it into a full-fledged return. His last screen role to date was as a washed-up former movie star in the lackluster comedy "Troop Beverly Hills" (1989).