MacMillan played junior hockey for the St. Michael's Majors in 1960-61 and 1961-62, He then switched to the Neil McNeil Maroons in 1962-63. In 1963-64 and 1964-65 he played for St. Dunstan's University. In 1965 he joined the Canadian National Team and played for them until they disbanded in 1970. His NHL career began in 1970-71 with the Toronto Maple Leafs. He would also play with the Atlanta Flames and New York Islanders before retiring in 1978. He played in 446 NHL games and scored 74 goals and 77 assists.