 Born at "Mount Pleasant" near Loretto, Virginia, Garnett was schooled by private teachers as a child and later engaged in planting.He was a member of the Virginia House of Delegates in 1800 and 1801 before being elected a Democratic-Republican to the United States House of Representatives in 1804, serving from 1805 to 1809.There, he was a member of the grand jury that indicted former Vice President Aaron Burr for treason in 1807.