Gelb worked in government in the mid- to late 1960s and oversaw the Defense Department's secret project to assemble a history of American involvement in Vietnam. The study became known as the Pentagon Papers, which Gelb's future employer, the Times, would later publish in a groundbreaking series of articles in 1971.
He was a senior fellow at the Brookings Institution when he joined the Times as diplomatic correspondent in 1973. He served as assistant secretary of state in the Carter administration from 1977 to 1979 and rejoined the Times in 1981.
Gelb was national security correspondent, deputy editorial page editor, editor of the op-ed page and columnist during his Times career. He played a leading role on the team that won the Pulitzer Prize for Explanatory Journalism in 1986 for its series on the Strategic Defense Initiative, known as "Star Wars," undertaken by the Reagan administration.
He was president of the Council on Foreign Relations from 1993 to 2003. "Power is as vital today as ever in securing national interests," Gelb argued in his 2010 book "Power Rules: How Common Sense Can Rescue American Foreign Policy."
He continued in the book: "It remains the necessary means to all important international ends, the principal coin of the global realm. Power rules, still, and there still are rules on how to best exercise it."
Leslie Howard Gelb was born on March 4, 1937, in New Rochelle, New York. He earned a bachelor's degree in government from Tufts University in 1959, a master's in 1961 and a doctorate in 1964 from Harvard University, where he drew the attention of faculty member Henry Kissinger, an early mentor who would become secretary of state.
In an interview Saturday with the Times, Kissinger said of Gelb: "I thought he had an unusual perception of the intangibles that make the difference between success and failure in foreign policy. I respected him greatly whether he supported me or criticized me."
In addition to his wife, Gelb is survived by three children.