During high school and college, he worked various jobs, including radio hosting at WFOY-Radio, contributing as a sportswriter to the
St. Augustine Record, and announcing shows at
Marineland. He was ordained as a priest in 1959 in the Diocese of St. Augustine. He served as the chaplain of St. Augustine Catholic Church and Student Center in Gainesville, Florida. He also served as Director of
Mission Nombre de Dios in
St. Augustine, Florida from 1962 to 1967. It was during his directorship that St. Augustine's 400th anniversary was celebrated and the Great Cross commemorating the founding of the mission was installed. Gannon became a faculty member in the history and religion departments at the University of Florida in 1967, beginning a teaching career of nearly 40 years. He resigned from the priesthood in 1976 and began teaching full-time at the
University of Florida. Gannon directed the Institute for Early Contact Period Studies, which conducted research into the voyages of Christopher Columbus and early interactions between Europeans and Native Americans. He served as the Assistant Dean of the
College of Liberal Arts and Sciences from 1976 to 1984, and as Associate Dean beginning in 1984. He was appointed Distinguished Service Professor of History by the university in 1992. He retired as a professor emeritus of history in 1998, but continued teaching until 2003. Gannon was a prolific author of books and articles on American history, religion, and military history. In 1968, he was a
war correspondent in
Vietnam for the Catholic magazine
America and the National Catholic News Service. His 1990 book
Operation Drumbeat discusses the history of Germany's early
U-boats and became a national bestseller. Gannon served as a member and chairman of several organizations on the local, state, and national level including the
Historic St. Augustine Preservation Board from 1969 to 1997, serving as its chairman twice; from 1975 to 1980 and 1976–77. He was a member of the
National Park Service Advisory Board for Southeastern States from 1972 to 1977 and served as its chairman from 1976 to 1977. He was appointed to the
National de Soto Expedition Trail Commission by the U.S. Secretary of the Interior in 1992. Gannon was also a board member of
Marineland of Florida and
University of Florida Historic St. Augustine, Inc., a successor to the Preservation Board. ==Personal life and legacy==