Born Merle Johnson, Jr. in New York City, Donahue was the son of a retired stage actress and the manager of the motion picture department of General Motors. One evening, producer William Asher and director James Sheldon spotted him in a diner at Malibu and arranged for a screen test with Columbia Pictures. It was unsuccessful, and Donahue was in a car accident in which he drove off a road and plunged 40 feet down a canyon. Actress Fran Bennett introduced him to Henry Willson, who represented Rock Hudson, among others. Willson signed him and it was he who changed Johnson’s name to Troy Donahue. “At first they had Paris, the lover of Helen of Troy, in mind,” Donahue says. “But I guess they thought they couldn’t name me Paris Donahue because there was already a Paris, France, and Paris, Illinois.” He later added, “it took me five minutes to get used to” his new name. Donahue signed with Universal Studios in 1956. He established himself with uncredited roles in The Monolith Monsters and Man Afraid in 1957, leading to larger parts in several films and on TV shows. Donahue starred in Monster on the Campus, Live Fast, Die Young, and The Tarnished Angels, all in 1958. He won praise in a small role in Imitation of Life.

The big break of Donahue’s career came when he was cast opposite Sandra Dee in A Summer Place, made by Warner Bros in 1959. This role made him a major star, especially among teenage audiences. In 1960 he was named by Film Daily as one of the five “finds” of the year. He signed a contract with Warner Bros. and played several successive leading roles in films such as Parrish, Susan Slade, Rome Adventure, Palm Springs Weekend, and A Distant Trumpet. He was reportedly going to be cast in Splendor in the Grass but missed out to Warren Beatty. “I guess because I was blond, blue-eyed and tanned, people associated me with all those beach movies that were around then, even though I never did one,” he later said. “I was always the goody-goody, the guy who did what he was supposed to.” In 1962 he claimed he received 5,000–7,500 fan letters a week. The following year, exhibitors voted him the 20th most popular star in the USA. He was also very popular in Japan. From 1960 to 1962, Donahue starred with Van Williams, Lee Patterson, Diane McBain, and Margarita Sierra in the ABC/WB series, Surfside 6, set in Miami Beach, Florida. After Surfside 6 was canceled, Donahue joined the cast of another ABC/WB detective series, Hawaiian Eye for its last season from 1962 to 1963 in the role of hotel director Philip Barton, with Robert Conrad and Connie Stevens in the series lead. Donahue also had a brief tenure as a recording artist at the height of his fame in the early 1960s, releasing a handful of singles for Warner Bros. records, including “Live Young” and “Somebody Loves Me”. However, none of his recordings entered the Billboard Hot 100 list.

Donahue continued to act in films throughout the 1980s and into the late 1990s. He appeared in the feature Grandview USA shot in Pontiac, Illinois. “Crowds of teenage girls would swarm around C. Thomas Howell, and teenage boys around Jamie Lee Curtis. But the major celebrity was Donahue,” recalls director Randall Kleiser. “These women who had grown up with him as their heartthrob followed him everywhere.” However he never obtained the recognition that he had in the earlier years of his career. Donahue’s final film role was in the 2000 comedy film The Boys Behind the Desk, directed by Sally Kirkland. Donahue was married four times and had one child. His first marriage was to actress Suzanne Pleshette, with whom he had twice co-starred in films. They wed on January 5, 1964 in Beverly Hills. They divorced nine months later, that same year. On October 21, 1966, Donahue married actress Valerie Allen in Dublin, Ireland. They separated in April 1967, and she filed for divorce in April 1968, charging him with cruelty, divorcing in November 1968. Donahue’s third marriage was to executive secretary Alma Sharpe. They married on November 15, 1969 in Roanoke, Virginia. “I couldn’t take care of myself, and I knew this friend would take me under her wing,” he says. They divorced in 1972. Donahue’s fourth and final marriage was to land developer Vicki Taylor. They were married in 1979 and divorced in 1981. In his final years, Donahue was in a long-term relationship with mezzo-soprano Zheng Cao, to whom he was engaged and with whom he lived in Santa Monica, California. In 1958 Donahue was jailed for 15 days for speeding. In 1961 his one-time fiancée Lili Kardell sued him for damages, claiming he had hit her without provocation. On August 30, 2001, Donahue suffered a heart attack and was admitted to Saint John’s Health Center in Santa Monica. He died there on September 2 at the age of 65.