Hamill began to work as a reporter for the New York Post in 1960. The 1962–63 New York City newspaper strike led Hamill to start writing magazine articles. By the fall of 1963 he was a correspondent for the Saturday Evening Post, stationed in Europe. Hamill spent six months in Barcelona and five months in Dublin, and traveled Europe interviewing actors, movie directors, and authors, as well as ordinary citizens. In August 1964 he returned to New York, reported on the Democratic Convention in Atlantic City, and was briefly employed as a feature writer at the New York Herald Tribune. He began writing a column for the New York Post in late 1965, and by the end of that year was reporting from Vietnam.
Over the course of nearly forty years Hamill worked at the Post, the New York Daily News, the Village Voice, and New York Newsday. He served briefly as editor of the Post, and later as editor-in-chief-of the Daily News. His resignation from the latter position after eight months prompted a letter of protest signed by more than a hundred of the paper's writers. Hamill's more extensive journalistic pieces have been published in New York, The New Yorker, Esquire, Playboy, Rolling Stone, and other periodicals. He has written about wars in Vietnam, Nicaragua, Lebanon and Northern Ireland, and reported on America's urban riots of the 1960s. Hamill wrote about the New York underclass and racial division, most notably in an essay for Esquire magazine entitled Breaking the Silence. 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.
Two collections of his selected journalism have been published: Irrational Ravings and Piecework (1996). For the Library of America he edited two volumes of the journalism of A.J. Liebling. In 1998, he published an extended essay on contemporary journalism titled News is a Verb: Journalism at the End of the Twentieth Century.