His secretary of 53 years, Evelyn Christel, said he died in Stony Brook of natural causes a day after his 88th birthday.
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.
He also gained attention for his second marriage to actress and heiress Dina Merrill, daughter of financier E.F. Hutton and Marjorie Merriweather Post, heiress to the Post cereal fortune and one of the world’s richest women.
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.
“My father was a loving father, devoted friend, dedicated professional and honorable man,” daughter Stephanie Saunders said in a statement. “He stood by his family, friends, and colleagues through good times and bad. He made a difference in all our lives and made our world a better place. We will all miss him terribly.”
Robertson had created a string of impressive performances in television and on Broadway, but always saw his role played in films by bigger names. His TV performances in “Days of Wine and Roses” and “The Hustler,” for example, were filmed with Jack Lemmon and Paul Newman, respectively. Robertson’s role in Tennessee Williams’ play “Orpheus Descending” was awarded to Marlon Brando in the movie.
Robertson first appeared in the “Charly” story in a TV version, “The Two Worlds of Charlie Gordon.” Both were based on “Flowers for Algernon,” a short story that author Daniel Keyes later revised into a novel. Robertson was determined that this time the big-screen role would not go to another actor.
“I bought the movie rights to the show, and I tried for eight years to persuade a studio to make it,” he said in 1968. “Finally I found a new company, ABC Films. I owned 50 percent of the gross, but I gave half of it to Ralph Nelson to direct.”
Critic Roger Ebert called Robertson’s portrayal “a sensitive, believable one.” The motion picture academy agreed, though Robertson was unable to get a break from an overseas movie shoot and was not on hand when his Oscar was announced.
Another memorable movie role, portraying future President Kennedy in the World War II drama “PT-109,” presented other challenges.
Released in 1963, it was the first movie to be made about a sitting president, and dozens of actors were considered. Kennedy himself favored Robertson, but he warned him he didn’t want someone trying to imitate his distinctive New England accent.
“That was fine with me,” the actor commented in 1963. “I think it would have been a mistake for me to say ‘Hahvahd’ or try to reproduce gestures. Then the audience would have been constantly aware that an actor was impersonating the president.”
He added that the film obviously couldn’t be done with heroics, “like Errol Flynn gunning down 30 of the enemy. This young naval officer just does things because they have to be done.”
After seeing photos of Robertson in costume, Kennedy had one critique: His hair was parted on the wrong side.
The actor dutifully trained his hair to part on the left.
“PT-109” was plagued with problems from the start: script changes, switch of directors, bad weather, snakes and mosquitoes in the Florida Keys where it was filmed.
The troubles were evident on the screen, and critics roundly rapped the film, although Robertson’s work won praise.
In 1977, Robertson made the headlines again, this time by blowing the whistle on a Hollywood financial scandal.
He had discovered that David Begelman, president of Columbia Pictures, had forged his signature on a $10,000 salary check, and he called the FBI and the Burbank and Beverly Hills police departments. Hollywood insiders were not happy with the ugly publicity.
“I got phone calls from powerful people who said, ‘You’ve been very fortunate in this business; I’m sure you wouldn’t want all this to come to an end,’ ” Robertson recalled in 1984.
Begelman served time for embezzlement, but he returned to the film business. He committed suicide in 1995.
Robertson said neither the studios nor the networks would hire him for four years.
He supported himself as a spokesman for AT&T until the drought ended in 1981 when he was hired by MGM for “Brainstorm,” Natalie Wood’s final film.
Born Sept. 9, 1923, in La Jolla, Calif., Robertson was 2 when he was adopted by wealthy parents who named him Clifford Parker Robertson III. After his parents divorced and his mother died, he was reared by his maternal grandmother, whom he adored.
Robertson studied briefly at Antioch College, majoring in journalism, then returned to California and appeared in two small roles in Hollywood movies. Rejected by the services in World War II because of a weak eye, he served in the Merchant Marine.
He set his sights on New York theater, and like dozens of other future stars, profited from the advent of live television drama. His Broadway roles also attracted notice, and after avoiding Hollywood offers for several years, he accepted a contract at Columbia Pictures.
“I think I held the record for the number of times I was on suspension,” he remarked in 1969. “I remember once I turned down a B picture, telling the boss, Harry Cohn, I would rather take a suspension. He shouted at me, ‘Kid, ya got more guts than brains.’ I think old Harry might have been right.”
Robertson’s first performance for Columbia, “Picnic,” was impressive, even though his screen pal, William Holden, stole the girl, Kim Novak. He followed with a tearjerker, “Autumn Leaves,” as Joan Crawford’s young husband, then a musical, “The Girl Most Likely” with Jane Powell. In 1959 he endeared himself to “Gidget” fans as The Big Kahuna, the mature Malibu surf bum who takes Gidget under his wing.
He remained a busy, versatile leading man through the ‘60s and ‘70s, but lacked the intensity of Brando, James Dean and others who brought a new style of acting to the screen.
“I’m not one of the Golden Six,” he commented in 1967, referring to the top male stars of that day. “I take what’s left over.”
“They all know me as a great utility player. ‘Good old Cliff,’ they say. Someday I’d like to be in there as the starting pitcher.”
The chance came with “Charly,” but after the usual Oscar flurry, he resumed his utility position.
Robertson had the most success in war movies. His strong presence made him ideal for such films as “The Naked and the Dead,” “Battle of Coral Sea,” “633 Squadron,” “Up From the Beach,” “The Devil’s Brigade,” “Too Late the Hero” and “Midway.”
He had a passion for flying, and he poured his movie earnings into buying and restoring World War I and II planes. He even entered balloon races, including one in 1964 from the mainland to Catalina Island that ended with him being rescued from the Pacific Ocean.
In 1957, Robertson married Lemmon’s ex-wife, Cynthia Stone, and they had a daughter, Stephanie, before splitting in 1960. In 1966, he married Merrill and they had a daughter, Heather. The couple divorced in 1989.
Robertson’s funeral is set for Friday in East Hampton.