Jack Fleming was born in Brooklyn, New York, in 1900. He graduated from Cornell University in 1922, where he was a classmate and friend of the late E. B. White. At Cornell, Fleming was editor of the Cornell Countryman, the school's agriculture journal.
In 1922, he became a reporter for the Springfield (Massachusetts) Union. Within 10 months, he was the city editor. He worked there until 1923, when he moved to Columbus, Ohio, to become extension news editor at Ohio State University.
He moved to Washington, D. C., in 1930, as a special assistant to Agriculture Secretary Arthur Hyde. He later wrote speeches for Secretary Henry Wallace. While at the USDA, he was also director of economic information and chief of the Bureau of Agricultural Economics.
In 1941, he became deputy director of the wartime Office of Facts and Figures, which later became the Office of War Information (OWI). A year later, he was named deputy director of OWI.
In 1945, he joined the staff of U.S. News as an editor for business economics and foreign news. In 1946, when World Report was started, Fleming was named editor of the new magazine. U. S. News and World Report merged in 1948, and Fleming became editor of the international news staff. He retired from the magazine in 1966. In retirement he served on President John F. Kennedy's Commission on Rural Poverty and was the principal author of the commission's report. He was also active in the Georgetown Presbyterian Church and was a member of the Cosmos Club in Washington.
He died in 1986 at the age of 86 in Silver Spring, Maryland.