The eldest of seven children of Catholic immigrants from Belfast, Northern Ireland, Pete Hamill was born in the Park Slope section of Brooklyn. His father, Billy Hamill, lost a leg as the result of an injury in a semi-pro soccer game in Brooklyn. Hamill's mother, Anne Devlin, a high school graduate in Belfast, arrived in New York on the day the stock market crashed in 1929. Billy Hamill met Anne Devlin in 1933 and they married the following year. Billy Hamill had jobs as a grocery clerk, in a war plant, and later in a factory producing lighting fixtures. Anne Hamill was employed in Wanamaker's department store, and also worked as a domestic, a nurses' aide, and a cashier in the RKO movie chain. His brother Denis would also grow up to become an award-winning columnist for the Daily News.
Hamill attended Holy Name of Jesus grammar school and delivered the Brooklyn Daily Eagle when he was 11. In 1949, Hamill attended the prestigious Regis High School in Manhattan, but left school when he was 15 to work as an apprentice sheet metal worker in the Brooklyn Navy Yard; 59 years later, in June 2010, Regis awarded him an honorary diploma. Inspired especially by the work of Milton Caniff, he was set on becoming a comic book artist. Hamill attended night classes at the School of Visual Arts (then called the Cartoonists and Illustrators School), with the goal of becoming a painter. In the fall of 1952, he enlisted in the U.S. Navy. After his discharge, in 1956-57, he was a student at Mexico City College on the G.I. Bill. Hamill has also lived in Spain, Ireland, San Juan, Puerto Rico, Rome, Los Angeles, and Santa Fe, New Mexico.
A friend of Robert F. Kennedy, Hamill helped persuade the senator to run for the United States presidency, then worked for the campaign and covered it as a journalist. He was one of four men who disarmed Sirhan Sirhan of his gun in the aftermath of the Robert F. Kennedy assassination.