John Francis Mercer, born on May 17, 1759, was the fifth of nine children born to John and Ann Mercer of Stafford County, VA. He attended the College of William and Mary, and in early 1776 he joined the 3d Virginia Regiment. Mercer became Gen. Charles Lee's aide-decamp in 1778, but after General Lee's court-martial in October 1779, Mercer resigned his commission. He spent the next year studying law at the College of William and Mary and then rejoined the army, where he served briefly under Lafayette