Born in Brunson, South Carolina, Mcmillan attended the public schools, Confederate Home College, Charleston, South Carolina, and Flora MacDonald College, Red Springs, North Carolina.

Mcmillan was elected as a Democrat to the Seventy-sixth Congress by special election, on November 7, 1939, to fill the vacancy caused by the death of her husband, Thomas S. McMillan, and served from November 7, 1939, to January 3, 1941.

She was appointed information liaison officer for the Department of State, Washington, D.C., on January 1, 1946, and served until July 31, 1957.

McMillan resided in Barnwell, South Carolina, until her death on November 8, 1976.

She was interred in Magnolia Cemetery, Charleston, South Carolina.