He lived with his life partner, Mark Miller (November 22, 1926 – June 9, 2015), whom he met in 1947 while they were 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.

During World War II he served in the US Navy as a communications officer in the Pacific Theatre of Operations from 1943 to 1946.

Nader moved into regular television roles in the late 1950s, appearing in several short-lived series, including The Further Adventures of Ellery Queen (1959) and The Man and the Challenge (1959–60). In the 1961–62 season, he appeared as insurance investigator Joe Shannon in the syndicated crime drama Shannon, co-starring with Regis Toomey.

He had small parts in You're in the Navy Now (1951), The Prowler (1951), Take Care of My Little Girl (1951), The Desert Fox: The Story of Rommel (1951), and Two Tickets to Broadway (1951). He had a bigger part in a Tim Holt Western, Overland Telegraph (1951), and a drama, Monsoon (1952). He was going to star in a film called GI Smith, but it was never made. He had unbilled bit roles in the studio films Phone Call from a Stranger (1951) and Down Among the Sheltering Palms (1952).

Nader's 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 as "one of the worst films ever made," it was financially successful and led to more prominent roles in other films. He supported Paulette Goddard in Sins of Jezebel (1953) and had a supporting role in Carnival Story (1954). He was the male love interest for Miss Robin Crusoe (1954) at Fox.

His rugged good looks won him a contract with Universal Studios, for which he made a number of films, although he often found himself struggling in the Shadow of more famous leading men such as Rock Hudson, Tony Curtis, and Jeff Chandler. His first film for Universal was a Western, Four Guns to the Border (1954), wherein he was billed beneath Rory Calhoun and Colleen Miller. He followed it with Six Bridges to Cross (1955), supporting Tony Curtis and Julie Adams in a role that Chandler had turned down.

Nader was promoted to lead in The Second Greatest Sex (1955) opposite Jeanne Crain and in Lady Godiva of Coventry (1955) opposite Maureen O'Hara, stepping in for Chandler again. In 1955, he won a Golden Globe Award for "Most Promising Newcomer."

He starred opposite Virginia Mayo in Congo Crossing (1956) and was second-billed to Chandler in Universal's expensive war epic Away All Boats (1956). He was Esther Williams's leading man in The Unguarded Moment (1956), which starred a young John Saxon. He had top billing in Four Girls in Town (1957) and Man Afraid (1957). Nader supported Audie Murphy in Joe Butterfly (1957), a military comedy. He had the lead in Appointment with a Shadow (1958) and Flood Tide (1958). He was Hedy Lamarr's love interest in The Female Animal (1958), replacing John Gavin. He had the starring role in Nowhere to Go, a 1958 British crime drama featuring the screen debut of Maggie Smith.

During this period, he also did episodic television and starred in several series, including NBC's The Man and the Challenge (1959–60). He is remembered for his starring role in "one of the worst films ever made", low-budget 3-D sci-fi film Robot Monster (1953).

Nader had the title role in a European swashbuckler, The Secret Mark of D'Artagnan (1963). He made Zigzag (1963) in the Philippines and The Great Space Adventure (1964) for Albert Zugsmith. He starred in The Human Duplicators (1965) and regularly guest-starred on TV shows. Nader went to Germany to star as FBI agent Jerry Cotton in the German film Tread Softly (1965). It was a hit and led to a series of films: Manhattan Night of Murder (1965), Tip Not Included (1966), The Trap Snaps Shut at Midnight (1966), Murderers Club of Brooklyn (1967), Death in the Red Jaguar (1968), Death and Diamonds (1968), and Dead Body on Broadway (1969).

In Europe he also appeared in The Million Eyes of Sumuru (1967) and The House of 1,000 Dolls (1967). One of his last films was Beyond Atlantis (1973), made in the Philippines.

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).

Discreetly gay during his film career, he and his life partner were among Rock Hudson's closest friends. He later wrote Chrome (1978), a science-fiction novel dealing openly and positively with a same-sex relationship.

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 cardiopulmonary failure, pneumonia, and multiple cerebral infarctions.

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.