Born in New York City to Irish-American parents and raised in Queens and the Bronx, Cahill was educated by Jesuits and studied ancient Greek and Latin. He continued his study of Greek and Latin literature, as well as medieval philosophy, scripture and theology, at Fordham University, where he completed a B.A. in classical literature and philosophy in 1964, and a pontifical degree in philosophy in 1965. He went on to complete his M.F.A. in film and dramatic literature at Columbia University in 1968.
In anticipation of writing The Gifts of the Jews, Cahill studied scripture at Union Theological Seminary in New York, and spent two years as a Visiting Scholar at the Jewish Theological Seminary of America, where he studied Hebrew and the Hebrew Bible. He also reads French and Italian. In 1999, he was awarded an honorary doctorate from Alfred University in New York.
Cahill has taught at Queens College, Fordham University, and Seton Hall University, served as the North American education correspondent for the Times of London, and was for many years a regular contributor to the Los Angeles Times Book Review. Prior to retiring to write full time, he was the Director of Religious Publishing at Doubleday for six years. He and his wife, Susan, also an author, divide their time between New York and Rome.