On 8 March 2008, he was just 15-years-old when he won a Harty Cup medal after lining out at left corner-forward in a 1-11 to 0-07 defeat of Thurles CBS in the final.
[2] On 19 April 2008, Mahony again lined out at left corner-forward when De La Salle College again faced Thurles CBS in the All-Ireland final.
On 1 March 2014, he scored 0-04 from frees when he lined out at centre forward when Waterford IT defeated the Cork Institute of Technology by 0-17 to 0-12 to win the Fitzgibbon Cup.
[4] On 1 March 2015, Mahony was selected at centre-forward when Waterford IT faced the University of Limerick in the Fitzgibbon Cup final.
[6] Mahony joined the Ballygunner club at a young age and played in all grades at juvenile and underage levels.
Lining out at left corner-forward against De La Salle, he scored 0-09 overall and won a third championship medal after the 2-12 to 2-09 victory.
Mahony was appointed captain of the Ballygunner under-21 team for the 2011 season and lined out in a third successive Under-21 A Championship final on 20 November 2011.
Mahony scored 1-07 but ended on the losing side for the second time in his career following a 3-16 to 3-13 defeat by first-time champions Passage.
[13] On 18 October 2015, Mahony won a fourth Waterford Senior Championship medal following a 0-16 to 0-12 defeat of Tallow in the final.
[16] On 22 October 2017, Mahony won a sixth Waterford Senior Championship medal as joint-captain when Ballygunner defeated De La Salle by 2-18 to 0-16 in the final.
He made his debut in that grade on 1 June 2011 when he lined out at centre-forward and scored six points from frees in a 4-12 to 1-16 defeat by Tipperary.
[29] On 15 July 2012, Mahony lined out in a second successive Munster final when Waterford faced Tipperary for the second year in-a-row.
[30] On 3 May 2015, Mahony won a National League medal after top-scoring with 0-11 from centre-forward in Waterford's 1-24 to 0-17 defeat of Cork in the final.
[32] On 1 May 2016, Mahony was introduced as an 88th-minute substitute for Michael Walsh when Waterford drew 0-22 apiece with Clare in the National League final.
[34] On 10 July 2016, Mahony lost the third Munster final of his career after top-scoring with 0-07 when Waterford suffered a 5-19 to 0-13 defeat by Tipperary.
He scored two points after coming on as a 58th-minute substitute for John Gardiner in the 3-14 to 1-16 defeat by Leinster in the semi-final of the Interprovincial Championship.