Peck died at his Los Angeles home overnight, with his wife, Veronique, at his side, spokesman Monroe Friedman said.
"She told me very briefly that he died peacefully. She was with him, holding his hand, and he just went to sleep," Friedman said. "He had just been getting older and more fragile. He wasn't really ill. He just sort of ran his course and died of old age."
Peck played many screen characters in his long career, from the romantic lead in "Roman Holiday" to the evil Nazi in "The Boys From Brazil."
But he is probably best be remembered for his portrayals of honorable men. Whether it was the idealistic lawyer in "To Kill A Mockingbird" or the or the reporter exposing prejudice in "Gentleman's Agreement," Peck was the epitome of quiet courage and moral strength.
During his first five years in films, Peck scored four Academy Award nominations as best actor: "Keys of the Kingdom" (1944), "The Yearling" (1946), "Gentleman's Agreement" (1947), "Twelve O'Clock High" (1949).
"Gentleman's Agreement," in which he played a magazine writer who poses as a Jew to expose anti-Semitism, was considered a daring film in its time. Peck commented in 1971 that his agent cautioned him: "You're just establishing yourself, and a lot of people will resent the picture. Anti-Semitism runs very deep in this country."
Peck ignored his advice. "Gentleman's Agreement" proved a moneymaker and won the Oscar as best picture.
CBS News Correspondent Jerry Bowen reports Peck was loved and respected for his ability to inhabit any character, no matter what role he played.
Years after playing the part of Atticus Finch in "To Kill A Mockingbird," Peck said playing the role brought him closest to being the kind of man he aspired to be.
Peck was revered both within and outside the Hollywood community for his choice of challenging roles in films like "To Kill A Mockingbird" and "Gentleman's Agreement."
In 1991 Peck was honored by the Kennedy Center and in 1992 received the Lincoln Center Lifetime Achievement Award.
"If maybe five, six, or seven times in my very long career the pictures had something to say and people could carry home with them, and added to the point of view that's woven into a dramatic story, something they can chew on a bit and maybe change their attitude toward a social issue ... well I like that," he once said.
The actor listed "Gentleman's Agreement" among his favorites of his movies. The others: the sea adventure "Captain Horatio Hornblower"; "Roman Holiday" in which he played a reporter to Audrey Hepburn's princess; "The Guns of Navarone" ("good, all-out entertainment, though it's really a comedy"); and "To Kill a Mockingbird" — for which he won the 1962 Oscar as best actor. He played Atticus Finch, a small-town Southern lawyer who defies public sentiment to defend a black man accused of rape.
"I put everything I had into it — all my feelings and everything I'd learned in 46 years of living, about family life and fathers and children," he remarked in 1989. "And my feelings about racial justice and inequality and opportunity."
In 2003, an American Film Institute listing of the top heroes in film history ranked Peck's Finch as No. 1.
In his 60s and 70s, movie roles grew sparse. He appeared as a U.S. president in "Amazing Grace and Chuck" (1987), maverick author Ambrose Bierce in "Old Gringo" (1989) and as a humane company owner victimized by a hostile takeover in "Other People's Money" (1991).
In 1993 he starred in a made-for-TV movie, "The Portrait," with Lauren Bacall, his co-star of "Designing Woman" (1957), and his daughter Cecilia.
A 1998 TV miniseries version of "Moby Dick" cast Peck in the small role of the preacher Father Mapple. He had played the protagonist, Ahab, in the 1956 film version.
"I'm working as much as I like," he commented in 1989. "I don't want to do, if I can avoid it, anything mediocre. It's kind of unseemly at my age to come out in a turkey."
Peck's lonely, disjointed childhood was the kind that often contributes to the making of actors. He was born Eldred Gregory Peck on April 5, 1916, in La Jolla, Calif. "My mother had found `Eldred' in a phone book, and I was stuck with it," he said.
The mother was a lively Missourian, the father was a quiet druggist, son of an Irish immigrant mother. His parents divorced when their son was 6. His next two years were divided between them, then he spent two years with his maternal grandmother in La Jolla. At 10 he was shipped off to a Roman Catholic military academy in Los Angeles where he was indoctrinated by "tough Irish nuns and square-jawed ROTC officers."
Peck majored in English at the University of California at Berkeley and rowed on the crew. One day he was accosted by the director of the campus little theater who said he was looking for a tall actor for an adaptation of "Moby Dick."
"I don't know why I said yes," he recalled in a 1989 interview. "I guess I was fearless, and it seemed like it might be fun. I wasn't any good, but I ended up doing five plays my last year in college."
Dropping the name of Eldred, he headed for New York after graduation with $195 in his pocket. He studied with Sanford Meisner and Martha Graham, worked as a barker at the 1939 World's Fair and as a tour guide at NBC. After summer stock and a tour with Katherine Cornell in "The Doctor's Dilemma," he made his Broadway debut is the lead in Emlyn Williams' "Morning Star."
A half-century later he remembered opening night:
"In the dressing room I gave myself a kick and said, `Get out there!' I was jittery for the first five minutes, and then I wasn't jittery anymore. You can die up there and say, `Call it off, give 'em their money back and let 'em go home.' Or you can collect yourself and do it."
The play flopped, but Peck's performance brought interest from Hollywood. He accepted a modest film, "Days of Glory," a story of Russian peasants during the Nazi invasion, mostly to use the $10,000 salary to pay off his dentist and other creditors. Then Darryl Zanuck offered him "Keys of the Kingdom."
Soon Peck was under non-exclusive contracts to four studios; he refused an exclusive pact with MGM despite Louis B. Mayer's tearful pleading. With most of the male stars absent in the war, the studios desperately needed strong leading men. Peck was exempt from service because of an old back injury.
A Roosevelt New Dealer, Peck campaigned for Harry Truman in 1948 "at a time when nobody thought he had a chance to win." He continued championing liberal causes, producing an anti-Vietnam War film in 1972, "The Trial of the Cantonsville Nine" and helping the campaign against the nomination of Robert Bork to the Supreme Court in 1987.
Rumors arose periodically that Peck planned to run for office. They started when Ronald Reagan defeated Edmund G. "Pat" Brown for governor of California in 1966. Brown cracked: "If they're going to run actors for governor, maybe the Democrats should have run Greg Peck."
"I never gave a thought to running," Peck always replied. "Not even in my heart of hearts do I have an ambition to do that."
Peck married his first wife, Greta, in 1942 and they had three sons, Jonathan, Stephen and Carey. Jonathan, a TV reporter, committed suicide at the age of 30. After their divorce in 1954, he married Veronique Passani, a Paris reporter. They had two children, Anthony and Cecilia, both actors.