O'Mahony was still eligible for the minor grade when he replaced Alan Hickey as first-choice goalkeeper on the Cork under-21 hurling team in 1992. He won a Munster medal the following year after a 1-18 to 3-09 defeat of Limerick in the final. O'Mahony made his senior debut for Cork on 14 November 1993, replacing Paul O'Callaghan at half-time in a National League game against Galway at Páirc Uí Chaoimh. He made one other appearance during the league but failed to be included on the Cork panel for the Munster Championship. In 2015, O'Mahony was added to Kieran Kingston's Cork senior hurling management team. As a goalkeeping coach he worked closely with Anthony Nash and Patrick Collins. On 9 July 2017, O'Mahony was part of the management team that guided Cork to the Munster title following a 1-25 to 1-20 defeat of Clare in the final. Following John Meyler's appointment as manager of the Cork senior hurling team in 2017, O'Mahony was added to the management team as a selector. On 1 July 2018, the new management team helped Cork to a second successive Munster title following a 2-24 to 3-19 defeat of Clare in the final.