Competing in ISSF 25 meter rapid fire pistol, he won an Olympic gold medal in 1960, was part of the American winning team at the 1952 World Championships, and won several gold and silver medals at the Pan American Games between 1955 and 1979.

[2] His professional career began in the United States Marine Corps, with McMillan eventually reaching the rank of lieutenant colonel.

[1] Between 1952 and 1976, he competed in the 25 m rapid fire pistol event in six Olympics, missing only the 1956 Games due to malfunctioning of his weapon during the US trials.

[citation needed] From 1974 to 1981, he worked as a Weapons Training Coordinator at San Diego County, California,[1] where he was injured in a shooting accident at the old Camp Elliot range.

In 1978, the Marine Corps established the McMillan Trophy for sports shooting in his honor.