John DeFrancis was born in Bridgeport, Connecticut in a family of modest Italian immigrant origins. His father, a laborer (who changed his name from DeFrancesco), died when DeFrancis was a young child. His mother was illiterate.

After graduating from Yale University in 1933 with a B.A. in economics, DeFrancis sailed to China with the intent of studying Chinese and working in business. In 1935, he accompanied H. Desmond Martin, a Canadian military historian, on a several-thousand-mile trip retracing the route of Genghis Khan through Mongolia and northwestern China. His book In the Footsteps of Genghis Khan (University of Hawai'i Press, 1993) describes this journey riding camels across the Gobi Desert, visiting the ruins of Khara-Khoto and rafting down the Yellow River. Along the way, he met the Chinese Muslim Ma Clique warlords Ma Buqing and Ma Bukang. DeFrancis returned to the United States in 1936 and did not visit China again until 1982.

DeFrancis began graduate studies in Chinese, first at Yale under George A. Kennedy and then at Columbia University due to Columbia's larger graduate program in Sinology. He received an M.A. from Columbia in 1941, then a Ph.D. in 1948 with a dissertation entitled "Nationalism and Language Reform in China", which was published by Princeton University Press in 1950. He began his academic career teaching Chinese at Johns Hopkins University during the period of McCarthyism and the Red Scare, but was blacklisted for defending his colleague Owen Lattimore from unsubstantiated allegations of being a "Russian spy." DeFrancis eventually returned to teaching, notably at Seton Hall University from 1947 to 1954, and the University of Hawai'i at Mānoa from 1966 to 1976. In the 1960s, at the request of John B. Tsu, he wrote a 12-volume series of Mandarin Chinese textbooks and readers published by Yale University Press (popularly known as the "DeFrancis series"), which were widely used in Chinese as a foreign language classes for decades; DeFrancis was one of the first educators outside China to use pinyin as an educational aid, and his textbooks are said to have had a "tremendous impact" on Chinese teaching in the West. He served as Associate Editor of the Journal of the American Oriental Society from 1950 to 1955 and the Journal of the Chinese Language Teachers Association from 1966 to 1978.

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