Garnett was the son of Colonel Robert McMillan, an Irish immigrant from Antrim who raised a company of infantry at Clarkesville, Georgia that would become Co. K of the 24th Georgia Regiment of Volunteers.
Young Garnett served as a Lt. in Company K, while his father became the commanding officer of the regiment.
He would serve in this position until June 1863 when he was promoted and transferred to a sharpshooters battalion.
During the American Civil War McMillan had served in the Georgia 3rd Battalion, Sharpshooters, commanding Company B.
He was elected in 1874 as a Democrat to represent Georgia's 9th congressional district in the 44th United States Congress.