Donald Saunders told The New York Times his father-in-law, who lived in Water Mill, N.Y., died at Stony Brook University Medical Center one day after his birthday.
Robertson, who won his Oscar in 1969, began his movie-making career in 1955 when he appeared in "Picnic." He also played future president John F. Kennedy in "PT 109."
Robertson had found himself persona non grata in Hollywood for almost four years after blowing the whistle on studio boss David Begelman as a forger and embezzler in 1977, the Times said. Begelman was fined and placed on probation after pleading no contest grand theft charges but became Metro-Goldwyn-Mayer top executive in 1980.