Early in his career, Cahill taught at several institutions in the New York area, including Queens College and Seton Hall University. He and his wife, Susan Cahill, opened a mail-order book company, and he worked as the advertising director at the New York Review of Books.
In 1990, he became director of religious publishing at Doubleday. He began pitching the Hinges of History series and How the Irish Saved Civilization around the same time. At a sales conference, he mentioned the idea to Talese, who immediately bought the series. It would occupy the remainder of his career, though Cahill took time off in 2002 to publish a short biography of Pope John XXIII as well as A Saint on Death Row: The Story of Dominique Green (2009).