William McMillan (March 2, 1764 - May, 1804) was a Delegate to the United States Congress from the Territory Northwest of the River Ohio. He was born near Abingdon, Washington County, Virginia. He graduated from the College of William and Mary, Williamsburg, Virginia. He studied law and then moved to Fort Washington (now Cincinnati, Ohio) in 1787. He was admitted to the bar in 1788 and commenced practice in Cincinnati. He became the first justice of the Court of General Quarter Sessions in 1790.
McMillan was a member of the Territorial House of Representatives in 1799 and 1800. He was elected to the Sixth Congress to fill the vacancy caused by the resignation of William H. Harrison and served from November 24, 1800, to March 3, 1801. He declined renomination in 1800. After the admission of Ohio into the Union in 1803 he was appointed United States District Attorney for Ohio, but owing to declining health did not assume the duties. He died in Cincinnati, Ohio, in May 1804. He was buried in Spring Grove Cemetery.