Schlesinger graduated from Harvard University with a B.A. in 1964, and earned his J.D. from Harvard Law School in 1968. During 1970, he began publishing, with other former supporters of Robert F. Kennedy and Eugene J. McCarthy, The New Democrat, a monthly magazine dedicated to uniting "the left and radical wings" and replacing the "dead leadership" in the Democratic Party. The magazine was critical of Democratic National Committee chairman Larry O'Brien, and promoted the candidacy of South Dakota Senator George McGovern over that of Maine Senator Ed Muskie and former Vice President Hubert Humphrey during the 1972 Democratic presidential primaries. Later, he worked as a staff writer for Time magazine.

Schlesinger served as a speechwriter and foreign policy advisor for New York Governor Mario Cuomo, who was elected during 1982 to the first of three consecutive terms. After Cuomo's defeat in 1994, Schlesinger worked for the United Nations Human Settlements Programme (UN-HABITAT, a United Nations agency for human settlements planning) before accepting a job running the World Policy Institute (WPI). He stepped down from the WPI in June 2006.

Schlesinger's book, "Bitter Fruit", published in 1982, about the 1954 US coup in Guatemala, has sold more than 100,000 copies. His subsequent study of the UN's founding, "Act of Creation", published in 2003, is the only authoritative account of the 1945 San Francisco Conference that drafted the UN Charter. It won the 2004 Harry S. Truman Book Award. In 2007, with his brother, Andrew, he edited his father's journals which cover the period from 1952-2000 and were published to wide acclaim.

Among other media accomplishments, Schlesinger has appeared in five documentaries on the United Nations and one on the 1954 Guatemalan coup d'état. Schlesinger is a regular contributor to many publications, including, among others, the Los Angeles Times, The Nation Magazine, Foreign Affairs, The New York Observer and The World Policy Journal. He blogs on Huffingtonpost.com and Maximsnews.com.