He has spent virtually his entire career at Yale. After graduating from the Pingry School in 1950, he received his bachelor's degree from Yale College in 1954, where he served as chairman of the Yale Daily News. In 1961, he earned his PhD in history from Yale. In over forty years of teaching at the university, he chaired the Department of History, served as master of Pierson College and directed the Yale Center for International and Area Studies. He retired from his appointment in 2000. 
Smith has spent more than fifteen years writing a history of the university. Yale in the 20th Century was to be published in August 2007, but it has yet to be released. He continues to teach the occasional seminar at Yale.
Smith received several awards from Yale College for his work there:
- 1986 - The William Clyde DeVane Medal for distinguished scholarship and teaching, awarded by the Yale Chapter (Alpha of Connecticut) of Phi Beta Kappa
- 1989 - The Harwood F. Byrnes-Richard B. Sewall Prize for Teaching Excellence
- 1997 - The Mory's Cup for service to the University
- American Diplomacy in the Second World War
- Morality, Reason and Power: American Diplomacy in the Carter Years
- The Aims of American Foreign Policy
- Dean Acheson
- Britain's Clandestine Submarines: 1914-1915
- The Last Years of the Monroe Doctrine: 1945-1993
- Ball, Molly (21 April 2000). "Gaddis Smith sails off to retirement after 50 years". Yale Herald. Retrieved 24 April 2014.
- Hill, Tyler; Mangino, Andrew (28 January 2008). "After 11 years of toil, Gaddis Smith nears completion of Yale history". Yale Daily News. Retrieved 23 April 2014.
- George W. Bush, Decision Points, London: Virgin Books, 2010, p. 14
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