Born in Los Angeles, California, McMillen competed for the United States in the 1952 Summer Olympics held in Helsinki, Finland in the 1500 m, where he won the silver medal with a rush at the end.[3] His final sprint missed catching leader Josy Barthel by a foot and a half at the line. Both athletes were given the same time, a new Olympic record.
McMillen is a 2004 inductee into the Glendale College Athletic Hall of Fame. He is also a member of the Occidental College Track and Field Hall of Fame.[4]