In early 1939 Mayne graduated from Queen's and joined George Maclaine & Co in Belfast, having been articled to TCG Mackintosh for the five previous years. Mayne won praise during the three Ireland matches he played in 1939, with one report stating "Mayne, whose quiet almost ruthless efficiency is in direct contrast to O'Loughlin's exuberance, appears on the slow side, but he covers the ground at an extraordinary speed for a man of his build, as many a three quarter and full back have discovered." His legal and sporting careers were cut short by the outbreak of the Second World War.