Before making the code switch to AFL, Mullin played Gaelic football for Kilmaine and the Mayo county team.

On 12 October 2019, Mullin was in the half back line as Kilmaine went up against Castlebar Mitchels B in the final of the Mayo Junior Football Championship.

[2] On 19 September 2020, Mullin was in midfield as Kilmaine faced Kilmeena in the final of the Mayo Junior Football Championship.

[4] Mullin didn't feature for the rest of the championship and Mayo went on to lose the All-Ireland final to Kildare.

[7] Mullin joined the senior squad in 2020, and made his championship debut in a first round win over Leitrim the same year.

[11] Mullin was later selected at corner back on the All Star team, and also claimed the Young Footballer of the Year award.