[1] Mahon was born on 25 December 1922 in Killurin, Killeigh, County Offaly, Ireland.
Mahon studied Canon Law in St. Patrick's College, Maynooth earning a doctorate in 1951.
He joined the teaching staff of the Saint Patrick's Society for the Foreign Missions, Kiltegan.
[3] Dr Mahon left Kiltegan in 1959 when he was appointed to the Roman Catholic Diocese of Ogoja, Nigeria, where he was Head of St. Thomas’ Teacher Training College.
Mahon moved to Kenya, after the outbreak of the Nigerian Civil War.