[4] On 28 March, Mullen won a second successive All-Ireland medal when he lined out at right wing-forward when St. Kieran's College defeated Ardscoil Rís from Limerick by 1-15 to 1-13.
On 26 February, he won a third successive Leinster Championship medal after scoring ten points in a 1-14 to 0-13 defeat of Kilkenny CBS in the final.
[6] On 25 March, Mullen top scored with 2-02 when St. Kieran's College suffered a 3-13 to 3-11 defeat by Our Lady's Secondary School from Templemore in the All-Ireland final.
[9] Mullen joined the Ballyhale Shamrocks club at a young age and played in all grades at juvenile and underage levels.
On 28 October 2018, Mullen scored a point from right wing-forward when Ballyhale Shamrocks defeated Bennettsbridge by 2-20 to 2-17 to win the Kilkenny Championship.
[13] On 27 October 2019, Mullen lined out at right wing-forward when Ballyhale Shamrocks faced James Stephens in the Kilkenny Championship final.
[17] Mullen ended the season by being included on the inaugural GAA Minor Star Awards Hurling Team of the Year.
[20] On 17 July 2019, Mullen won a Leinster Championship medal as an unused substitute when Kilkenny defeated Wexford by 1-17 to 0-18 in the final.
[23] Mullen was selected at right corner-forward for the All-Ireland final against Tipperary on 18 August 2019, however, his participation was in doubt after being hospitalised with an illness in the week leading up to the match.
[26] On 24 February 2020, it was confirmed that Mullen would miss the rest of the season having torn a knee cruciate ligament in Kilkenny's draw with Clare in Round 4 of the National League.