In 1964 McCarthy joined the Cork under-21 team. His first two seasons ended with first round exits from the championship, however, his appointment as captain of the team in 1966 coincided with an upturn in Cork's fortunes. He won a Munster medal that year as Cork trounced Limerick by 5–12 to 2–6. The subsequent All-Ireland final ended in a draw as Wexford recorded 5–6 to Cork's 3–12. The replay also ended all square – 4–9 apiece. At the third time of asking Cork emerged victorious with a huge tally of 9–9 to 5–9. This victory gave Cork their first All-Ireland title in this grade and gave McCarthy an All-Ireland Under-21 Hurling Championship medal while also the honour of collecting the cup.