McMillan was the chairman and CEO of Permian Exploration Corporation, an oil exploration company, the chairman of the First Federal Savings Bank in Roswell, New Mexico, and the founder and CEO of McMillan Production Company. He was appointed in 2001 to be the chairman of the Sallie Mae Corporation by President George W. Bush.
McMillan served in the New Mexico House of Representatives from 1971 to 1982. He ran for the United States Senate seat in New Mexico in 1994 but lost to incumbent Democrat Jeff Bingaman by a 54% to 46% margin. In 2000, McMillan was the New Mexico state chairman of George W. Bush's presidential campaign.
McMillan was survived by his wife, Kay, four children, and eight grandchildren.