Sean McMahon was born in Derry in 1931 and he educated at St Derry's Columb's College and in Queen's University in Belfast. He returned to Derry to teach in St Columb's and taught Mathematics until his retirement in 1988.
During his teaching years he staged many productions of works by Shakespeare and modern dramatists both for St Columb's and for amateur theatre groups in Derry. He also wrote the lyrics for musical shows.
His literary career properly began with his anthology The Best from the Bell, which was followed by A Book of Iirish Quotations and Rich and Rare, an anthology of prose and verse. Over the past twenty years he has written and edited dozens of books, including biographies of Ulster writers Sam Hanna Bell and Robert Lynd, the bestselling A Short History of Ireland and books on all aspects of Irish life and culture, including A Short History of Ulster, The Island of Saints and Scholars and Irish Names for Children.