McMillan's father is Melvin Guenther, a railroad worker.
McMillan graduated from Mount Baker High School.
[1] McMillan attended East Kootenay Community College.
McMillan was defeated for renomination in the Republican primary in 2016 by Priscilla Giddings, who went on to win her seat.
[2] On March 30, 2017, McMillan announced her intentions via social media that she planned to run for her seat again in 2018.