McCarthy made his senior debut for Cork during the 1964–65 National Hurling League, however, it was 1966 before he made his championship debut in a Munster quarter-final draw with Clare. McCarthy remained on the starting fifteen for the rest of the season and also took over the captaincy from Peter Doolan. A subsequent 4–9 to 2–9 defeat of Waterford gave him his first Munster medal. This victory allowed Cork to advance directly to the All-Ireland final where arch-rivals Kilkenny provided the opposition. "The Cats" played with the wind in the first-half but only led by two points at the interval thanks to a Colm Sheehan goal for Cork. Sheehan scored a second after the break while John O'Halloran netted Cork's third goal to defeat an Eddie Keher-inspired Kilkenny by 3–9 to 1–10. Not only was it a first championship for Cork in twelve years, but it was McCarthy's first All-Ireland medal while he also had the honour of collecting the Liam MacCarthy Cup.