Alfred Hitchcock considered him for the Sam Loomis part in Psycho (1960), but the part went to John Gavin. Robert Wise considered him for the lead role in The Sand Pebbles (1966), but that part went to Steve McQueen.
Stephanie Robertson, his daughter with Cynthia Stone, was born in 1959. His daughter with Dina Merrill, Heather Robertson, was born in 1968 and died of cancer in 2007.
A political liberal, he personally campaigned for Congressman Mo Udall in the New Hampshire Democratic Presidential primary in 1976.
After serving as a merchant marine, he studied at Antioch College in Yellow Springs, Ohio. He left the college without getting his degree. He moved to New York City where he studied at the Actors Studio.
After the Japanese attack on Pearl Harbor, eighteen year-old Cliff - then serving on a merchant ship in the Pacific - was reported dead to his family in California.
Along with Leonard Nimoy, David McCallum, Barbara Rush and Peter Breck, he is one of only five actors to appear in both "The Outer Limits" (1963) and the revival "The Outer Limits" (1995).
Both he and his then wife Dina Merrill played "Special Guest Villains" in "Batman" (1966).
He has two roles in common with Martin Sheen. He and Sheen played John F. Kennedy in PT 109 (1963) and "Kennedy" (1983) respectively. They also played Ben Parker in Spider-Man (2002) and The Amazing Spider-Man (2012) respectively.
He was a longtime resident of Water Mill, New York at the time of his death. His daughter, Heather Robertson, died in 2007. He is survived by his daughter, Stephanie Robertson Saunders, of Charlestown, South Carolina and son-in-law, Donald Saunders, and a grandchild.
He was interred at Cedar Lawn Cemetery in East Hampton, Suffolk, New York.
His father, whom he was never close to, was described as "an idle heir" to family money made in ranching. His parents divorced when he was 1, and his mother died only a year later, so Robertson was raised by his paternal grandmother and an aunt.
His first wife was Jack Lemmon's ex.
In 1972, he said that 'Nobody made more mediocre films than I did,' including Too Late the Hero (1970) which he described as 'a bunch of junk.'.
Inducted into the National Aviation Hall of Fame in 2006 in the Advocate category.
Member of the jury at the Berlin International Film Festival 1989
Owns a number of vintage aircraft, including an original German Messerschmitt ME-108, which is on display at the Parker/O'Malley Air Museum in upstate New York.
Passed up the chance to play Dirty Harry (1971).
Personally chosen by John F. Kennedy to portray a World War II US Navy Lieutenant Kennedy in 'PT 109' (1963).
Son of Clifford Parker Robertson II and the former Audrey Winningham Robertson. His parents divorced in 1924. His mother died in 1925. He was raised by his maternal grandmother and an aunt.
Special guest at Roger Ebert's 4th annual Overlooked Film Festival in Champaign, Illinois in March 2002.