Mullen was born in Courtown Harbour, Co. Wexford and educated at Belvedere College and the Royal College of Surgeons in Ireland. He played as hooker, winning 25 caps for Ireland from 1947 to 1952. He captained the Irish team to their first Grand Slam in the 1948 Five Nations Championship and was one of eight players from that team who lived to see the country's next Grand Slam in 2009.