Mayne's first full Ireland rugby cap also came in 1937, in a match against Wales. After gaining five more caps for Ireland as a lock forward, Mayne was selected for the 1938 British Lions tour to South Africa. While the Lions lost the first test, a South African newspaper stated Mayne was "outstanding in a pack which gamely and untiringly stood up to the tremendous task". He played in seventeen of the twenty provincial matches and in all three tests. On returning from South Africa, he joined Malone RFC in Belfast.