Kagan graduated from Yale University in 1980, where he studied history and founded the Yale Political Monthly. He later earned an MPP from the John F. Kennedy School of Government at Harvard University and a PhD in US history from American University in Washington, D.C. He is a senior fellow at the Carnegie Endowment for International Peace and adjunct professor of history at Georgetown University.

In 1983, Robert Kagan was foreign policy advisor to New York Representative Jack Kemp. Between 1984 and 1986, he worked at the State Department Policy Planning Staff and was a speechwriter for Secretary of State George P. Shultz. From 1986 to 1988, he served in the Bureau of Inter-American Affairs at the State Department.

In 1997, Kagan co-founded the Project for the New American Century,notably co-signing an open letter to President Clinton on Iraq.

Kagan is a Senior Associate with the Carnegie Endowment for International Peace. He was a foreign policy advisor to John McCain, the Republican Party's nominee for President of the United States in the 2008 election. Foreign policy: 2 camps seek McCain's ear - International Herald Tribune

Kagan is a member of the Council on Foreign Relations.

Kagan is a columnist for the Washington Post and is syndicated by the New York Times Syndicate. He is a contributing editor at both The New Republic and the Weekly Standard, and has also written for the New York Times, Foreign Affairs, the Wall Street Journal, Commentary, World Affairs, and Policy Review. His book, Of Paradise and Power, was a national and international bestseller and has been translated into 25 languages. His book, Dangerous Nation, won the 2007 Lepgold Prize from Georgetown University. He is listed by Foreign Policy and Prospect as one of the world's "Top 100 Public Intellectuals."

Robert Kagan is the son of Yale classical historian and author, Donald Kagan. He is married to Victoria Nuland, the former U.S. ambassador to NATO, and has two children.

He is the brother of political commentator Frederick Kagan.