In 1962, Mullan became a referee in the Scottish Division One. Three years later, he was appointed as a FIFA referee.
In 1972, Mullan was selected as a referee for UEFA Euro 1972, where he officiated a semi-final match between Belgium and West Germany.
Later that year, Mullan was selected as a referee for the Summer Olympics in West Germany, where he officiated two matches, both in Munchen: Hungary - Brazil 2-2 (29 August) and F.R. Germany - German Democratic Republic 2-3 (8 September).
In 1973, Mullan retired from refereeing.
(Wikipedia)