Fleming earned his Ph.D. in medieval literature from Princeton University in 1963. Later he expanded his doctoral dissertation into the publication The Roman de la Rose. A Study in Allegory and Iconography (1969). His fields of expertise included medieval English, French, and Latin literatures, and the history and culture of the Franciscan Order in the Middle Ages. He dealt with the latter topic in the book An Introduction to the Franciscan Literature of the Middle Ages (1977). He is perhaps best known in Princeton for his popular and erudite lecture course on Geoffrey Chaucer. In 2006, after 40 years at Princeton, he moved to emeritus status. In retirement Fleming has expanded his field of scholarly publication beyond medieval subjects. His first trade book was a study of classic anti-Communist literature entitled The Anti-Communist Manifestos. This was followed by The Dark Side of the Enlightenment. He returned to formal scholarly publication in 2017 with a monograph on the Portuguese Renaissance poet: Luís de Camões. The Poet as Scriptural Exegete and he continues to publish occasional commissioned essays and book reviews. He is introduced to the Czech reading public for the first time.