His father, grandfather and two of his brothers were blacksmiths, a motif he incorporated into his coat of arms as a bishop.
Mahony was at the Gort sheep fair when he heard the call of God to become a priest, joining the Society of African Missions in 1936.
Following completing his secondary education at Sacred Heart College, Ballinafad, he went to St. Josephs Wilton, Cork and began his university studies in University College Cork, before moving to the SMA Novitiate at Cloughballymore, Kilcolgan, Co Galway, for two years, studying philosophy and completing his undergraduate degree in University College Galway, earning a B.A.
He entered the SMA major seminary in Dromantine House, Newry, studying theology, and being ordained a priest in St. Colman's Cathedral, in 1947.
In the latter year, he was appointed Prefect Apostolic of Illorin (which was carved out of the diocese of ondo),[3] a province of more than thirty thousand square miles and a population of about one million.