His secretary of 53 years, Evelyn Christel, says he died Saturday afternoon in Stony Brook, N.Y., of natural causes.
Robertson never elevated into the top ranks of leading men, but he remained a popular actor from the mid-1950s into the following century. His later roles included kindly Uncle Ben in the "Spider-Man" movies.
His triumph came in 1968 with his Academy Award performance in "Charly," as a mentally disabled man who undergoes medical treatment that makes him a genius -- until a poignant regression to his former state.
His funeral is set for Friday in East Hampton.