Born Eldred Gregory Peck in La Jolla, California, he was the son of a Missouri mother and a chemist called Gregory Peck, whose mother, Catherine Ashe, was an Irish immigrant from County Kerry. Catherine Ashe was related to the Irish patriot Thomas Ashe, who took part in the Irish Easter Rising in the year of Peck's birth and who died on hunger strike in 1917. Peck's parents divorced when he was five and he was reared by his grandmother. Peck was sent to a Roman Catholic military school in Los Angeles at the age of 10. When he graduated, he went to San Diego State University, but dropped out a year later. For a short time, he took a job driving a truck for an oil company. In 1936, he enrolled as a pre-med student at the University of California, Berkeley. He majored in English and rowed on the university crew. He was recruited by the school's Little Theater and appeared in five plays his senior year.
After graduation, Peck dropped the name "Eldred" and headed to New York City in 1939 to study at the Neighborhood Playhouse. He was often broke and sometimes slept in Central Park. He worked at the 1939 World's Fair and as a tour guide for NBC television. He made his Broadway debut as the lead in Emlyn Williams' "Morning Star" in 1942. Peck's acting abilities were in high demand during World War II, since he was exempt from military service due to a back injury suffered while receiving dance and movement lessons from Martha Graham as part of his acting training. Twentieth Century Fox claimed he had injured his back while rowing a boat at university. In Peck's words, "In Hollywood, they didn't think a dance class was macho enough, I guess. I've been trying to straighten out that story for years."
Peck's first film was Days of Glory, released in 1944. He was nominated for the Academy Award for Best Actor five times, four of which came in his first five years of film acting for Keys of the Kingdom (1944), The Yearling (1946), Gentleman's Agreement (1947), Twelve O'Clock High (1949).
In 1947 while many Hollywood figures were being blacklisted for similar activities, he signed a letter deploring a House Un-American Activities Committee investigation of alleged communists in the film industry. He was outspoken against the Vietnam War, while remaining supportive of his son, Jonathan, who was fighting there. In 1972, Peck produced the film version of Philip Berrigan's play, The Trial of the Catonsville Nine about the prosecution of a group of Vietnam protesters for civil disobedience.
In the 1980s he moved to television, where he starred in the mini-series The Blue and the Gray, playing Abraham Lincoln. He also starred in a TV film, The Scarlet and The Black, about a real-life Catholic priest in the Vatican who smuggled Jews and other refugees away from the Nazis during World War II.
Peck retired from active film-making in the early 1990s, having received a Lifetime Achievement Award from the American Film Institute in 1989. A lifelong supporter of the Democratic Party, he was suggested once as a possible Democrat candidate to run against Ronald Reagan for the post of Governor of California. In an interview with the Irish media, Peck revealed that former President of the United States Lyndon Johnson had told him that had he sought re-election, he intended to offer Peck the post of US ambassador to Ireland, a post Peck, on account of his Irish ancestry, said he might well have taken saying "it would have been a great adventure".
In 2000, he was made a Doctor of Letters by the National University of Ireland. He was a founding patron of the University College Dublin School of Film, where he persuaded Martin Scorsese to become an honorary patron.
He died in his sleep at the age of 87 in his Los Angeles home, with his second wife, Veronique, at his side. He was survived by Veronique, their two children and one of his children from his earlier marriage. His oldest son, Jonathan, had killed himself in 1975.