Nader was born in Pasadena, California, the son of Alice (née Scott), who was from Kansas, and George G. Nader, who was from Illinois. During World War II he served in the US Navy as a communications officer in the Pacific Theatre of Operations
He began his film career in 1950, after earning his Bachelor of Arts in theatre arts at Occidental College. He appeared in several productions at the Pasadena Playhouse, which led to a number of bit parts in 1951 and 1952. His first starring role was in Robot Monster (1953), a 3-D feature film directed by Phil Tucker. Although the film is remembered primarily for its "camp" attributes it was financially successful, this role and his rugged good looks won him a Universal Studios contract, for whom he made a number of films. In 1955, he won a Golden Globe Award for "Most Promising Newcomer."
Nader often found himself struggling in the shadow of more famous leading men, such as Rock Hudson, Tony Curtis, and Jeff Chandler. His films of that period included 1954's Carnival Story and Sins of Jezebel and 1956's Away All Boats. He also was Esther Williams' leading man in her first straight dramatic film, The Unguarded Moment which also starred a young John Saxon, released by Universal in 1956. Nader had the starring role in Nowhere to Go, a British crime drama featuring the screen debut of Maggie Smith.
He moved into television in the late 1950s, appearing in several short-lived series including The Further Adventures of Ellery Queen and The Man and the Challenge. In the 1961–1962 season, he appeared as insurance investigator Joe Shannon in the syndicated crime drama Shannon; his co-star was Regis Toomey. Nader also appeared frequently on The Loretta Young Show, a dramatic anthology series on NBC.
He moved to Europe, where he found steady work in films. His most notable recurring role during this period was as FBI agent Jerry Cotton in a German film series, becoming one of the most popular American film stars in Germany.
In the 1970s, Nader suffered an eye injury in an automobile accident, which made him particularly sensitive to the bright lights of movie sets, and forced him to retire from acting. He began writing, including his 1978 science fiction novel Chrome, which dealt with a forbidden romance between a man and an android (also male).
According to Variety's Army Archerd, Nader had completed a book called The Perils of Paul, about the gay community in Hollywood, which he did not want published until after his death.
Although Nader was not openly gay during his film career, he generally didn't feign relationships with women to conceal it, instead deflecting that he hadn't met "the right one". He lived with his life partner Mark Miller (November 22, 1926 – June 9, 2015), whom he met in 1947 while acting in a play together. Miller worked as Rock Hudson's personal secretary from 1972 until the star's death, and the couple inherited the interest from Hudson's estate after his death from AIDS complications in 1985. Nader publicly acknowledged his sexual orientation shortly afterward. Hudson biographer Sara Davidson described Nader, Miller, and another person as "Rock's family for most of his adult life".
Nader and Miller eventually returned to the U.S. and settled in Palm Springs. Stricken by multiple medical problems, Nader entered the hospital in September 2001. He died at Woodland Hills, California of cardio-pulmonary failure, pneumonia, and multiple cerebral infarctions. Nader was survived by Miller (with whom he had spent 55 years), his cousins Sally Kubly and Roberta Cavell, and his nephew, actor Michael Nader.
His ashes were scattered at sea; a cenotaph in his honor, together with Mark Miller and Rock Hudson, exists in Cathedral City's Forest Lawn Cemetery. In 2002, a Golden Palm Star on the Palm Springs, California, Walk of Stars was dedicated to him.