His father, Cornelius Joseph Sheehan, worked as a dairy farmer; his mother, Mary (O'Shea), was a housewife.
In 1963, during the Buddhist crisis, Sheehan and David Halberstam debunked the claim by the Ngô Đình Diệm regime that the Army of the Republic of Vietnam regular forces had perpetrated the Xá Lợi Pagoda raids, which U.S. authorities initially accepted.
In 1964, he joined The New York Times and worked the city desk for a while before returning to the Far East, first to Indonesia and then to spend another year in Vietnam.
[4] Sheehan was one of numerous U.S. and international journalists who received valuable information from Pham Xuan An, a 20-year veteran correspondent for Time Magazine and Reuters, later revealed to also be a spy for the National Liberation Front for South Vietnam.
After being notified of their existence by Marcus Raskin and Ralph Stavins at the Institute for Policy Studies, Sheehan copied the Pentagon Papers for the Times on March 2, 1971,[6][7][8][9][10] against leaker and Vietnam-era acquaintance Daniel Ellsberg's wishes.
[12] Sheehan published an article in the New York Times Book Review on March 28, 1971, entitled "Should We have War Crime Trials?".
He suggested that the conduct of the Vietnam War could be a crime against humanity and that senior U.S. political and military leaders could be subject to trial.
Marcus Aurelius Arnheiter, the subject of the book, proceeded to bring an action for libel against Sheehan but was ultimately unsuccessful.
[14] Sheehan then secured an unpaid leave from the Times to work on a book about John Paul Vann, a dramatic figure among American leaders in the early stages of the war in Vietnam.
[16] Although he received an advance of $67,500 (of which he was entitled to $45,000 prior to publication) from Random House in 1972, Sheehan – a "dreadfully slow" writer who "[chased after] the last fact" – mainly subsisted on lecture fees and fellowships from the John Simon Guggenheim Memorial Foundation (1973–1974), the Adlai Stevenson Institute of International Studies at the University of Chicago (1973–1975), the Lehrman Institute (1975–1976), the Rockefeller Foundation (1976–1977), and the Woodrow Wilson International Center for Scholars (1979–1980) for the remainder of the 1970s.
[16] Still beset by health problems (including a pinched nerve and osteoarthritis), he eventually completed the book, A Bright Shining Lie: John Paul Vann and America in Vietnam, in 1986.