Kennedy was born in Wallsend, Tyne and Wear, and attended St. Cuthbert's Grammar School in Newcastle upon Tyne. Subsequently, he graduated with first class honors in history from Newcastle University and obtained his doctorate from St. Antony's College, Oxford. He was a member of the History Department at the University of East Anglia between 1970 and 1983. He is also a Fellow of the Royal Historical Society, a former Visiting Fellow of the Institute for Advanced Study in Princeton, New Jersey, United States and of the Alexander von Humboldt Foundation in Germany. In 2007-8, Kennedy was the Phillipe Roman Professor of History and International Affairs at the London School of Economics.
He is the J. Richardson Dilworth professor of British history at Yale University in New Haven, Connecticut, United States.
His most famous book, The Rise and Fall of the Great Powers, has been translated into 23 languages and assesses the interaction between economics and strategy over the past five centuries. The book was incredibly well received by fellow historians, with A.J.P. Taylor labelling it "An encyclopaedia in itself" and Sir Michael Howard crediting it as "a deeply humane book in the very best sense of the word". His most recent book is The Parliament of Man, in which he contemplates the past and future of the United Nations.
He is on the editorial board of numerous scholarly journals and writes for The New York Times, The Atlantic, and many foreign-language newspapers and magazines. His monthly column on current global issues is distributed worldwide by the Los Angeles Times Syndicate/Tribune Media Services. He was made a Commander of the British Empire in 2001. The National Maritime Museum awarded him its Caird Medal in 2005 for his contributions to naval history.