On 21 October 2016, Mannion scored a goal when Ahascragh-Fohenagh defeated Ballinderreen by 2–15 to 0–08 to win the Galway Intermediate Championship.

[2] On 18 February 2017, Mannion top scored for Ahascragh-Fohenagh with five points when they suffered a 2–15 to 0–06 defeat by Carrickshock in the All-Ireland final.

He won an All-Ireland medal as a non-playing substitute on 4 September following Galway's 1–21 to 1–12 defeat of Dublin in the final.

He made his first appearance for the team on 16 February 2014 and scored four points from right corner-forward in a 0–28 to 1–12 defeat of Dublin in the National League.

Mannion scored two points from left corner-forward in the 0–26 to 2–17 victory in what was Galway's first All-Ireland Championship in 29 years.

[17] On 1 July 2018, Mannion scored three points for Galway in their 0-18 apiece draw with Kilkenny in the Leinster final.

[18] The replay a week later saw him win his first Leinster Championship medal on the field of play after Galway retained the title with a 1–28 to 3–15 victory.