Ellsberg joined the U.S. Marine Corps in 1954. Over the next three years he served as a rifle platoon leader, operations officer, and rifle company commander.
In 1957 Ellsberg became a Junior Fellow in the Society of Fellows, Harvard University. He earned his Ph.D. in Economics at Harvard with his thesis, Risk, Ambiguity and Decision. In 1959, he became a strategic analyst at the RAND Corporation, and consultant to the Department of Defense and the White House, specializing in problems of the command and control of nuclear weapons, nuclear war plans, and crisis decision-making.
Ellsberg joined the Defense Department in 1964 as Special Assistant to Assistant Secretary of Defense (International Security Affairs) John McNaughton, working on Vietnam. He transferred to the State Department in 1965 to serve two years at the US Embassy in Saigon, evaluating pacification on the front lines. He worked under Edward Lansdale. Ellsberg liked Lansdale because of his commitment to democracy.
Ellsberg also agreed with Lansdale that the pacification program should be run by the Vietnamese. He argued that unless it was a Vietnam project it would never work. Lansdale knew that there was a deep xenophobia among Vietnamese. However, as he pointed out, he believed "Lyndon Johnson would have been just as xenophobic if Canadians or British or the French moved in force into the United States and took charge of his dreams for a great Society, told him what to do, and spread out by thousands throughout the nation to see that it got done."
In 1967 Ellsberg became a member of the McNamara Study Group that in 1968 had produced the classified History of Decision Making in Vietnam, 1945-1968. Ellsberg, disillusioned with the progress of the war, believed this document should be made available to the public. He gave a copy of what later became known as the Pentagon Papers to William Fulbright. However, he refused to do anything with the document, so Ellsberg gave a copy to Phil Geyelin of the Washington Post. Katharine Graham and Ben Bradlee decided against publishing the contents on the document.
Ellsberg now went to the New York Times and they began publishing extracts from the document on 13th June, 1971. This included information that Dwight Eisenhower had made a secret commitment to help the French defeat the rebellion in Vietnam. The document also showed that John F. Kennedy had turned this commitment into a war by using a secret "provocation strategy" that led to the Gulf of Tonkin incidents and that Lyndon B. Johnson had planned from the beginning of his presidency to expand the war.
Ben Bradlee was criticised by his journalists for failing to break this story. He now made attempts to catch up and on June 18, 1971, the Washington Post began publishing extracts from the History of Decision Making in Vietnam, 1945-1968. However, Bradlee concentrated on the period when Dwight Eisenhower was in power. The first story reported on how the Eisenhower administration had delayed democratic elections in Vietnam.
Richard Nixon now made attempts to prevent anymore extracts from the Pentagon Papers being published. The Supreme Court ruled against Nixon and Hugo Black commented that the two newspapers "should be commended for serving the purpose that the Founding Fathers saw so clearly".
Ellsberg's trial, on twelve felony counts posing a possible sentence of 115 years, was dismissed in 1973 on grounds of governmental misconduct against him, which led to the convictions of several White House aides and figured in the impeachment proceedings against President Richard Nixon.