Born in Brooklyn, New York, Newfield attended Boys High School and graduated from Hunter College with a degree in journalism in 1961. He was involved with SDS (Students for a Democratic Society) at the time it was founded in 1962, and was active supporter of anti-war, New Left politics in the sixties. He went to work for The Village Voice in 1964. His book, A Prophetic Minority, published in 1966, provides an account of early sixties sit-ins and de-segregation movement, creation of the Student Nonviolent Coordinating Committee (SNCC), involvement of white students in SNCC and voter registration drives in Mississippi, and rise of SDS. Newfield continued writing for The Village Voice until 1988, penning some 700 articles for the newspaper during his 24 years on its staff as columnist, reporter and senior editor. He was a columnist at the Daily News from 1988 to 1990 and at the New York Post from 1991 to 2001.

In January, 1969, Newfield published Robert Kennedy: A Memoir, considered by many to be the best book ever written about Robert Kennedy. Newfield's memoir was personal and passionate. While Newfield had a close relationship with Kennedy, he was tough-minded and determined to find the man within the myth...an unusual achievement for any Kennedy biographer. Newfield wrote:

(Robert Kennedy) was not ruthless, or an excessively ambitious politician, but a conflicted, vulnerable man, impatient with the small contrivances of politics. And he was not a divisive, unpopular figure, but rather a healing force. The root of my argument is that Robert Kennedy was the one politician of his time who might have united the black and white poor into a new majority for change -- an American liberalism hardly noticed.

In 1988, Newfield was writer, reporter, and co-producer of the acclaimed Discovery Network documentary, Robert Kennedy.

He was an activist, in addition to being a journalist. He was with New York Sen. Robert F. Kennedy when the latter was assassinated by Sirhan Sirhan in Los Angeles, in June, 1968.

Aside from providing exposes (in his columns in the Post) of abuse of power by government officials and by businessmen, he wrote a number of books. His assessment of the mayoralty of Rudy Giuliani, The Full Rudy won the American Book Award in 2003.

Newfield died in December 2004, from kidney and lung cancer.