John Francis Mercer was born on May 17, 1759, at Marlborough in Stafford County, Virginia, the son of the lawyer and planter John Mercer and his second wife, Ann Roy Mercer. He was the fifth of their nine children and part of a much larger blended family, because his father had children by an earlier marriage. The Mercers were prominent in law, land speculation, plantation agriculture, and Virginia politics. Their wealth depended upon enslaved labor.

Mercer’s father possessed a formidable legal library and reputation, but died in 1768 when John Francis was only nine. His mother died two years later. Private tutors educated the boy at home, giving him the classical preparation expected of a planter’s son destined for law and public service. Older half-brothers, including George and James Mercer, offered examples of military, legal, and political advancement.

Mercer entered the College of William and Mary, where James Monroe was among his contemporaries, and graduated in 1775 at about sixteen. Williamsburg placed him near Virginia’s courts, legislature, and revolutionary debate. He intended to study law, and later accounts associate his legal training with Thomas Jefferson, but armed conflict interrupted the normal progression from college to the bar.

On February 26, 1776, Mercer accepted a commission as first lieutenant in the Third Virginia Regiment. Despite his youth, family standing and education helped qualify him for officer rank. He learned drill, administration, supply, and battlefield command within the Continental Army and was promoted to captain in 1777. He fought at Brandywine, where he was wounded. In 1778 he became aide-de-camp to General Charles Lee with the rank of major, placing him close to high command, correspondence, and operational planning.

Mercer resigned after Lee’s court-martial and departure from the army, then returned to legal study before again serving with Virginia forces. His early life thus ended unusually quickly. By twenty, orphanhood, private tutoring, college, legal preparation, commissioned service, combat wounds, and staff duty had already formed the experience on which his later careers as lawyer, constitutional convention delegate, congressman, planter, and governor were built.