He suffered the heart attack on Thursday and was taken to a Santa Monica hospital, but he never recovered.
Donahue was the Freddie Prinze Jr. of his day--a matinee idol whose face sent many a girl into a tizzy, but whose films put many a critic to sleep.
He was born Merle Johnson Jr. on January 27, 1936, into a show-biz family. His dad ran the motion picture division of General Motors, and his mother was a wannabe actress.
It was only after a stint studying journalism at Columbia University that he decided to give acting a shot, making the rounds in local theater and then moving to Tinseltown at age 19. But the name Merle Johnson was deemed too pedestrian by Hollywood standards, and the blond-haired, blue-eyed actor was rechristened Troy Donahue by his agent, Henry Willson--the same guy who turned Roy Harold Scherer Jr. into Rock Hudson.
He got his big movie break in the 1957 flick Man Afraid. That caught the attention of Warner Bros., which signed him to a deal in 1959. He reached full-blown Hollywood hunkdom that same year in A Summer Place. (His onscreen relationship with costar Dee would later be immortalized in "Look at Me, I'm Sandra Dee" from the musical Grease. The song contains the lines "As for you, Troy Donahue / I know what you wanna do / You've got your crust, I'm no object of lust / I'm just plain Sandra Dee.")
Donahue starred in such films as Parrish (1961), Rome Adventure (1962) and My Blood Runs Cold (1964) before segueing to TV in the early '60s series Surfside 6 and Hawaiian Eye. He also had a bit part in 1974's The Godfather, Part II, playing--in a Hollywood inside joke--a character named Merle Johnson. But by then his career was in decline.
In fact, his life stuck pretty close to a True Hollywood Story script: unbelievable fame in his 20s, followed by failed marriages, drug and alcohol abuse and virtual anonymity. It was only after a year spent homeless in Central Park that he decided to get sober and try salvaging his life.
"He had some adversity in his life and challenged it all," frequent costar Connie Stevens said in a CNN interview Sunday.
A revitalized Donahue began appearing in low-budget, direct-to-video and indie films in the late 1980s and early 1990s, including such forgettable releases as Assault of the Party Nerds, Nudity Required and Bimbo Movie Bash. In 1990, he had a small part in John Waters' Cry-Baby. (Waters is a self-proclaimed "huge fan" of the actor.)
Donahue was married at least four times, including a brief union to actress Suzanne Pleshette, according to the Internet Movie Database. He is survived by a sister, two children and his fiancée, opera singer Zheng Cao.