McMillan's international debut was in 1985 at the Junior Curling World Championships in Perth , where he won his first precious metal with a bronze medal. At the EM in 1989 he became European champion for the first time. At the EM in 1994, 1995, 1996 and 1999 McMillan was able to win the title of European champion again. In 1999 he became world champion.
McMillan played a total of three times for the British team at the Winter Olympics. When it first took part in Calgary in 1988, the team came in eighth. In 1992 in Albertville , McMillander was Skip and finished in fifth place. In his third participation, also as Skip, in Salt Lake City in 2002 , he finished eighth.