Born in Shanballymore, County Cork, Mannix first played competitive Gaelic football at juvenile and underage levels with the Dr. Croke's amalgamation.
Mannix enjoyed a concurrent Gaelic football career with Clonakilty, culminating with the winning of a county senior championship.
[1] Mannix made his debut on the inter-county scene at the age of seventeen when he was selected for the Cork minor team.
He enjoyed one championship season with the minor team, culminating with the winning of an All-Ireland medal in 1981.
By this stage Mannix had also played for the Cork senior team, making his debut during the 1984–85 league.