 He served on the building department's board of examiners for 12 years, and was a member of the board of park commissioners and president of the board during the mayoral administration of William L. Strong.In 1906 McMillan was a resident of Mahopac, New York when he was elected to the United States House of Representatives as a Republican, and served in the 60th Congress (March 4, 1907 - March 3, 1909).He was not a candidate for renomination in 1908.