Mahon, the eldest of four children, was born into a prominent Roman Catholic family in Ennis, County Clare. His father was Patrick Mahon of New Park, who took part in the Rebellion of 1798; his mother Barbara, a considerable heiress, was the only daughter of James O'Gorman of Ennis. Mahon studied at Clongowes Wood College, where he was one of the earliest pupils, and at Trinity College Dublin, where he took his BA in 1822 and his MA in law in 1832. Until his father's death in 1821, he was sent an annual allowance of £500; afterwards, he inherited half the family property, also becoming a magistrate for Clare. He soon adopted the title "the O'Gorman Mahon", O'Gorman being his mother's maiden name. This was in part intended to give the false impression that he was the head of the Mahon clan.