Born Merle Johnson, Jr., Donahue was initially a journalism student at Columbia University before he decided to become an actor in Hollywood, where he was represented by Rock Hudson's agent, Henry Willson. According to Robert Hofler's 2005 biography, The Man Who Invented Rock Hudson: The Pretty Boys and Dirty Deals of Henry Willson, Willson tried out the name Troy on Rory Calhoun and James Darren, with no success, before it finally stuck to Donahue. The blond heartthrob established himself with uncredited roles in The Monolith Monsters and Man Afraid in 1957, leading to larger parts in several films.
He starred in Monster on the Campus, Live Fast, Die Young, and The Tarnished Angels, all in 1958, and opposite fellow teen idol Sandra Dee in A Summer Place in 1959. The latter made him a star, especially among teenage audiences. He signed a contract with Warner Bros. and played several successive leading roles in films such as Rome Adventure and A Distant Trumpet. The latter two co-starred Suzanne Pleshette, whom he married in 1964 but divorced later that same year.
After the release of My Blood Runs Cold (1965), Donahue's contract with Warner Bros. ended. He later struggled to find new roles and had problems with drug addiction and alcoholism. He was married again in 1966, to actress Valerie Allen, but they divorced in 1968. In 1970 he appeared in the daytime drama The Secret Storm. In 1974 he was cast in his most high profile role, a small part in The Godfather Part II as the new fiancé of Connie Corleone. His character was called Merle Johnson, Donahue's real name.
Donahue also starred in the 1960s television series Surfside 6.
Donahue spent his last few years with his fiancée, mezzo-soprano Zheng Cao. He died of a heart attack at the age of 65.
