Born in Scotland, McMillan joined the Cleveland Stars of the American Soccer League (ASL) in 1973.
[1] That season, he scored eleven goals and added seven assists in seven games to place second in the league's points list.
[2] In 1974, he jumped to the expansion Los Angeles Aztecs of the North American Soccer League (NASL).
[3] In 1978, McMillan was hired as the head coach of the American Soccer League Los Angeles Skyhawks.
In 1986, he opened Camp North American Soccer Academy in Commerce, Georgia.