Born in the town of Ulmer in Allendale County, McMillan received his early childhood education at the schools in Ulmer. He graduated from the Orangeburg Collegiate Institute in 1907 and taught school for the next two years in Perry. McMillan then enrolled at the University of South Carolina and graduated in 1912. In 1913, he completed the law course at the university and was admitted to the bar the same year. He moved to Charleston where he began the practice of law on January 1, 1915, with James B. Heyward, as well as pursuing his agricultural interests.
McMillan served in the South Carolina House of Representatives from 1917 to 1924 and he served as speaker from 1923 to 1924. In addition, he was the head baseball coach at The Citadel from 1916 to 1919; for five years before law school, he had played professional minor league baseball with the South Atlantic League.
McMillan's house in Charleston, South Carolina contributes to a local National Register historic district.
While serving in the United States Congress, McMillan maintained a house in Charleston, South Carolina at 171 Moultrie St. Today, the house is a contributing structure to the Hampton Park Terrace National Register Historic District and is used as a faculty house for the Citadel.
