Hersh was born in Chicago and graducated from the University of Chicago. He began his career in journalism as a police reporter for the City News Bureau in 1959. He later became a correspondent for United Press International in South Dakota. In 1963 went on to become a Chicago and Washington DC correspondent for the Associated Press. Five years later, Hersh was hired as a reporter for The New York Times Washington Bureau, where he served from 1972 to 1975 and again in 1979.
His work first gained worldwide recognition in 1969 for exposing the My Lai massacre and its cover-up during the Vietnam War, for which he received the Pulitzer Prize for international reporting.
His book The Price of Power: Kissinger in the Nixon White House won him the National Book Critics Circle Award and the Los Angeles Times book prize in biography. Hersh has written a total of eight books and contributed to the PBS television documentary, Buying the Bomb (1985).
Hersh currently contributes regularly to The New Yorker on military and security matters.