His few attempts to play a villain were considered unsuccessful, perhaps because the public could not accept Peck as anything other than good. He was considered too young at 38 (the movie was filmed in 1954) to play Captain Ahab in Moby Dick (1956), especially since the character was described in Herman Melville's novel as an old man. Peck admitted he only agreed to play Nazi Dr Josef Mengele in The Boys from Brazil (1978) because he wanted to work with Sir Laurence Olivier. Although the film and his performance were savaged by the critics, Peck remained loyal to it.
His mother died in May 1992 at the age of 97.
His paternal grandmother, Catherine Ashe, was an immigrant from County Kerry, Ireland. She was a relative of Thomas Ashe, an Irish patriot who fought in the Easter Rising in 1916 and died on hunger strike the following year.
His performance as Atticus Finch in To Kill a Mockingbird (1962) is ranked #13 on Premiere Magazine's 100 Greatest Movie Characters of All Time.
His picture appears on a nondenominated USA commemorative postage stamp in the Legends of Hollywood series, issued 28 April 2011. Peck is shown as the character Atticus Finch in To Kill a Mockingbird (1962). Price on day of issue was 44¢. First day of issue ceremonies were held at the Academy of Motion Picture Arts and Sciences.
Honorary chair, Los Angeles Library Foundation. 
In 1946 he met and befriended Gary Cooper, with whom he was often compared in terms of looks and acting style.
In 1947, at the beginning of the anti-communist investigations in Hollywood, Peck signed a letter deploring the witch hunts despite being warned his signature could hurt his career.
In 1948, amid the anti-Communist hysteria sweeping the country during the McCarthy "Red Scare" era, he was called before a "fact finding committee" set up by the California Legislature to ferret out alleged Communists and their sympathizers in the entertainment industry. He was summoned because of his association with a host of "liberal" organizations and causes, along with several other stars. He gave the committee a list of every organization to which he had contributed money, along with their letterheads, and said that he contributed to them because they were legitimate organizations. He told the committee, "I am not now and never have been associated with any communist organization or supporters of communism. I am not a communist, never was a communist and I have no sympathy with communist activities".
In 1987 he joined Burt Lancaster, Martin Sheen and Lloyd Bridges in narrating a TV commercial for the People for the American Way, opposing the confirmation of President Ronald Reagan's nominee to the Supreme Court, ultra-conservative judge Robert Bork. Bork, who came under intense criticism in part because of his past vociferous opposition to civil rights laws, ultimately failed to be confirmed by the Senate.
In 1996, veteran character actor Richard Jaeckel, Peck's costar in The Gunfighter (1950), was diagnosed with cancer, and Jaeckel's wife had Alzheimer's disease. The Jaeckels had lost their Brentwood home, were over $1 million in debt, and Jaeckel was basically homeless. His family tried unsuccessfully to enter him into Woodland Hills Motion Picture and Television Hospital. Peck lobbied for Jaeckel's admittance and he was treated within three days. Jaeckel stayed in the hospital until his passing in June 1997.
In 1997, as a presenter at the Gay and Lesbian Alliance Against Defamation (GLAAD) awards ceremony, he said, "It just seems silly to me that something so right and simple has to be fought for at all."
In 1999 he publicly berated Congress for failing to pass legislation preventing teenagers from buying guns, following the Columbine high school massacre.
In 1999 he supported the decision to give Elia Kazan an honorary Oscar for lifetime achievement, saying he believed that a man's work should be separate from his life.
In 1999, the American Film Institute named Peck among the Greatest Male Stars of All Time, ranking at No. 12.
In December 2002 Peck visited his wife in hospital in Los Angeles after she underwent surgery to relieve pressure on two vertebrae. The sight of the veteran actor in hospital sparked more press rumors that he was seriously ill.
In February 1955 Peck was set to star in The Proud Ones (1956), but the role was eventually played by Robert Ryan.
In his 80s his frail and thin appearance frequently sparked press rumors of his impending death, particularly when in 2001 he attended Jack Lemmon's funeral with his head bandaged from a recent fall.
In late November of 2005, thieves stole Peck's "Hollywood Walk of Fame" star using a cement saw to cut the bronze-and-terrazzo marker out of the sidewalk. In a simple ceremony, a new star honoring the late actor was unveiled on December 1st to replace the stolen one. Hollywood's honorary mayor Johnny Grant lifted a covering and announced, "Ladies and gentlemen, we proudly welcome back to the Hollywood Walk of Fame, Gregory Peck." Peck's star was the fourth to be stolen since the Walk of Fame was inaugurated. James Stewart's and Kirk Douglas' stars disappeared some years ago after being removed for construction and were later recovered by police in the nearby city of South Gate. Gene Autry's star also vanished during a construction project. A call saying it had been found in Iowa proved to be a false alarm.
In the 1950s, the Arizona-Sonora Desert Museum near Tucson, AZ, named one of their male javalinas "Gregory Peckory" in his honor; incidentally, their female was named "Olivia de Javalina" to honor actress Olivia de Havilland.