He was born to a Jewish family on March 24, 1930, in New York City. His work appeared in such periodicals as Harper's, The Atlantic, Esquire and The New Yorker. He was a war correspondent in Korea, Vietnam, Iraq, and Yugoslavia. He was also a correspondent and later a bureau chief of CBS News in Spain.

He also wrote ten books, including the controversial title, The Untold Story of Jewish Revenge Against Germans in 1945. The book caused an uproar because Sack reported that, at the end of World War II, a number of Jewish Holocaust survivors, like Salomon Morel, ran some Polish-Communist concentration camps and prisons, where they tortured and killed mostly German but also Polish civilians, including women and children.

John Sack did extensive research and fought off major attempts to suppress his book. He died on March 27, 2004, of complications from bone marrow cancer.