MacMillan began his National Hockey League career with the Toronto Maple Leafs in 1960. He also played with the Detroit Red Wings. Macmillan left the NHL following the 1965 season and retired from hockey altogether in 1971. He won 2 Stanley Cups with the Toronto Maple Leafs in 1962 and 1963.
After 1964-65 MacMillan played in the minor pros until he retired in 1971.