Minor and under-21

Mannion first lined out for Galway as a member of the minor team during the 2011 All-Ireland Championship. 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.^[4]

On 28 July 2012, Mannion made his first appearance for the Galway minor team. He scored two points from midfield in a 4–20 to 2–11 defeat of Wexford in the All-Ireland quarter-final.^[5]

Mannion was drafted onto the Galway under-21 team for the 2013 All-Ireland Championship. He made his first appearance for the team on 24 August and scored three points from full-forward in a 1–16 to 0–07 defeat by Clare.^[6] His three-year tenure with the under-21 team ended without success.^{[citation needed]}

Senior

Mannion was still eligible for the under-21 grade when he was selected for the Galway senior team. 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.^[7] Mannion was also included on Galway's starting fifteen for the Leinster Championship. He made his debut on 1 June in a 1–22 to 0–23 defeat of Laois.^[8]

On 5 July 2015, Mannion lined out in his first Leinster final. He scored a point from right corner-forward in the 1–23 to 2–17 defeat by Kilkenny.^[9] Mannion was switched to left corner-forward for the All-Ireland final against Kilkenny on 6 September. He was held scoreless in the 1–22 to 1–18 defeat.^[10] Mannion ended the season by receiving an All-Star award.^[11]

On 3 July 2016, Mannion scored five points in Galway's 1–26 to 0–22 defeat by Kilkenny in the Leinster final.^[12]

Mannion claimed his first silverware with the Galway senior team on 22 April 2017. He scored 1-01 when Galway won the National League title after a 3–21 to 0–14 defeat of Tipperary in the final.^[13] Mannion subsequently missed Galway's 0–29 to 1–17 defeat of Wexford in the Leinster final because of an ankle injury.^[14] He was back on the starting fifteen for the All-Ireland final against Waterford on 3 September. 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.^[15][16] He ended the season by being receiving a second All-Star nomination.^[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.^[19] On 19 August, Mannion was at left corner-forward when Galway faced Limerick in the All-Ireland final. He was held scoreless throughout the 3–16 to 2–18 defeat.^[20] Mannion ended the season by being nominated for a third All-Star award.^[21]