William Peter Hamill was born on June 24th 1935 in Park Slope, Brooklyn to parents Billy and Anne Hamill (née Devlin), immigrants from Belfast, North Ireland. Hamill attended P.S. 107 and Holy Name of Jesus elementary school, working as a delivery boy for the Brooklyn Daily Eagle to earn extra money. In 1949 Hamill was offered a scholarship to attend Regis High School in Manhattan. In 1951 he transferred to St. Agnes for a short time before dropping out for a job as a sheet metal worker at the Brooklyn Navy Yard. During this time, he moved out of his family's home and began taking night classes at the Cartoonists and Illustrators School (now the School of Visual Arts). In 1952, Hamill enlisted in the U.S. Navy, using money from his G.I. Bill to study painting at Mexico City College.

After returning to New York, he worked as a designer for the Greek magazine Atlantis, and soon opened his own studio. He also began writing letters to the editor of the New York Post, several of which were printed. In 1960, Post editor Jimmy Wechsler responded to one of Hamill's letters with a job offer, a three-month trial working the night desk. Hamill was hired, but a 1962 printers' union strike temporarily closed the paper, leaving Hamill to write freelance for several publications including the Saturday Evening Post. In February 1962 he married Ramona Negron, and their daughter Adriene was born later that year. In 1963, Hamill moved with his wife and daughter to Spain, where he worked as the European correspondent for the Saturday Evening Post, covering conflicts in Ireland and Lebanon. The couple's second daughter Deirdre was born in 1964.

In 1965 Hamill rejoined the New York Post and was sent to Vietnam to cover the war. In 1968 he joined the campaign trail of friend and democratic primary nominee Robert Kennedy. Hamill was standing a few feet from Kennedy when he was assassinated, and helped to subdue assassin Sirhan Sirhan. Later that year Hamill published his first novel, Killing for Christ. He worked as a columnist for the Post from 1969-1974, publishing a collection of his columns in Irrational Ravings (1971) and a second novel, The Gift (1973). Hamill was awarded a 1976 Grammy for his work writing the liner notes on Bob Dylan's album Blood on the Tracks. Hamill and Ramona Negron divorced in 1970.

From 1977-1979 Hamill worked as a columnist for the New York Daily News, as an editor for the Mexico City News in 1987 and as a columnist for Esquire from 1989-1991. He also contributed articles to New York Magazine, Rolling Stone, Playboy, and the Village Voice, bringing his work to a national audience. In 1987 he married journalist Fukikio Aoki. He continued to publish his own work, including Dirty Laundry (1978), Flesh and Blood (1977), The Deadly Piece (1979), The Invisible City: Short Stories (1980), Guns of Heaven (1984), Loving Women (1989) and Tokyo Sketches: Short Stories (1992).

In 1993 Hamill was named Editor of the New York Post, a position he held for five weeks. The following year his memoir, A Drinking Life, was published to positive reviews followed by a second collection of his columns called Piecework (1996.) In 1997 he was named Editor-in-Chief of the New York Daily News, which he resigned after eight months. In 1998 he published the bestselling Snow in August, followed by News is a Verb (1998) and biographies Why Sinatra Matters (1999) and Diego Rivera (1999).

In addition to books and articles Hamill was also a screenwriter, adapting stories from his novels and writing original content. Hamill briefly appears onscreen in several films and is credited as a writer on ten made for TV movies. He also gave lectures and participated in talks at numerous universities, libraries and organizations, speaking about his career and promoting projects. His ninth novel, Forever, was published in 2003, and in 2004 New York University named Hamill Distinguished Writer in Residence at the Arthur L. Carter Journalism Institute. Following the appointment he published his second memoir Downtown: My Manhattan, followed by novels North River (2007) and Tabloid City (2011).

Hamill's final full-length publication, a collection of short stories called The Christmas Kid: And Other Brooklyn Stories was released in 2012. He was the recipient of numerous awards including the Columbia Journalism Award for Lifetime Achievement, the Ernie Pyle Lifetime Achievement Award and the George Polk Career Award. Hamill died on August 5, 2020 following heart and kidney failure. He is interred at Green-Wood Cemetery. On June 24, 2021 7th Avenue between 11th and 12th Streets was renamed Pete Hamill Way, in honor of his childhood home at 378 Seventh Avenue.