Born in Clonmel, County Tipperary, Lonergan first played competitive Gaelic football during his schooling at CBS High School Clonmel. He arrived on the inter-county scene at the age of sixteen when he first linked up with the Tipperary minor team before later joining the under-21 side. He made his senior debut during the 2015 championship. Lonergan immediately became a regular member of the starting fifteen.[citation needed]
