The eldest of seven children of Catholic immigrants from Belfast, Northern Ireland, Hamill was born in the Park Slope section of Brooklyn.

[2] His father, Billy Hamill, lost a leg as the result of an injury during a semi-professional soccer game in Brooklyn.

Anne Hamill was employed in Wanamaker's department store, and she also worked as a domestic, a nurses' aide, and a cashier in the RKO movie chain.

[5] Hamill attended Holy Name of Jesus grammar school[6] and delivered the Brooklyn Daily Eagle when he was 11.

[13] In 1958, while serving as the art director for a Greek-language newspaper the Atlantis, Hamill talked his way into writing his first piece about his friend, Puerto Rican professional boxer José Torres, then a neophyte middleweight and Olympic champion.

[19] Hamill's more extensive journalistic pieces have been published in New York, The New Yorker, Esquire, Playboy, Rolling Stone, and other periodicals.

Hamill wrote about the New York underclass and racial division, most notably in an essay for Esquire magazine, "Breaking the Silence".

[21] He also wrote about boxing, baseball, art, and contemporary music, winning a Grammy Award in 1975 for the liner notes to Bob Dylan's Blood on the Tracks.

[22] However, while at the New York Post, Hamill defamatorally wrote of the later exonerated Central Park Five that the teens hailed “from a world of crack, welfare, guns, knives, indifference and ignorance…a land with no fathers…to smash, hurt, rob, stomp, rape.

His first novel, a thriller called A Killing for Christ, about a plot to assassinate the Pope on Easter Sunday in Rome, was published in 1968.

[2] Hamill's memoir Downtown: My Manhattan includes his reporting for the New York Daily News on the destruction of the World Trade Center on September 11, 2001, at which he was present.

[34] In his introduction to A Living Lens: Photographs of Jewish Life from the Pages of the Forward (2007), Hamill evokes the heyday of American Yiddish journalism.

[39] Among his writings on the subject are an introduction to Terry and the Pirates: Volume Two by Milton Caniff (2007),[40] and an introductory text for a revised version of Al Hirschfeld's The Speakeasies of 1932 (2003).

[42] Hamill penned a handful of teleplays and screenplays, including adaptations of his own novels, and had a few minor film roles, usually playing a generic "reporter," or himself.

[43] According to Robert Rosen, the Producer on French Connection II, he re-wrote all of the dialogue in the film, working nearly non-stop for three days, for which he did not receive screen credit.

His work meant that he resided for long periods of time in Spain, Ireland, Saigon, San Juan, Puerto Rico, Rome, Los Angeles, and Santa Fe, New Mexico.

[23][45] A friend of Robert F. Kennedy, Hamill helped persuade the senator to run for the United States presidency.