Robertson, who won a Best Actor Oscar for 'Charly' (1968) and blew the lid off a check-forging scandal at Columbia Pictures in 1977, died of natural causes, the Hollywood Reporter said.
Robertson's best-known role was, perhaps, in real-life. He was touted as a 'profile in courage' for reporting that Columbia studio head David Begelman had forged his name on a USD 10,000 check in the late 70s.
Robertson was considered a hero by many for, essentially, putting his career on the line by taking on a powerful studio head.
He also won an Emmy Award for his performance in 'The Game on Bob Hope Presents the Chrysler Theatre', and was nominated for an Emmy for 'The Two Worlds of Charly Gordon'. After performing in 'Charly Gordon', he bought the rights, and channelled it into an Academy Award-winning performance.
He began his movie career as a contract player at Columbia in the 1950s. Throughout a long acting career, Robertson mixed his acting between movies, TV and the stage, performing mainly as a character actor.
"I get bored playing the same type," he had said.
Robertson's most popular performance was as the young John F Kennedy in 'PT-109'. Kennedy, in fact, had suggested that he be cast to play the part. Robertson also starred as the chief executive of a large US corporation in Brainstorm. He also portrayed Playboy editor/publisher Hugh Hefner in Star 80.
Robertson was married to actress Dina Merrill from 1966 to 1986 and previously to Cynthia Stone Lemmon. He had one daughter from each marriage.