Schlesinger attended the Phillips Exeter Academy, New Hampshire, and received his first degree at the age of 20 from Harvard College, where he graduated summa cum laude in 1938. After spending the 1938–1939 academic year at Peterhouse, Cambridge as a Henry Fellow, he was appointed to a three-year Junior Fellowship in the Harvard Society of Fellows in the fall of 1939. At the time, Fellows were not allowed to pursue advanced degrees, "a requirement intended to keep them off the standard academic treadmill"; as such, Schlesinger would never earn a doctorate. His fellowship was interrupted by the United States entering World War II. After failing his military medical examination, Schlesinger joined the Office of War Information. From 1943 to 1945, he served as an intelligence analyst in the Office of Strategic Services, a precursor to the CIA.