Mercer was born on January 17, 1725, at Pitsligo Kirk, the family farm in Aberdeenshire, Scotland. His parents were William Mercer, a Presbyterian minister, and Ann Monro Mercer. By 1744, he had obtained a medical degree from the University of Aberdeen. Mercer and his family were Jacobites, meaning that they supported the restoration of the Stuarts to the English throne. The Stuart king, a Catholic, had been deposed in 1688 by the Dutch Protestant William III and his wife, Mary, who was King James’s daughter. Queen Anne, another of James’s children, ruled from 1702 to 1707, after which the Hanoverian kings succeeded to the throne. When Charles Edward Stuart, Anne’s brother, arrived in western Scotland, in July 1744, Mercer promptly joined the cause, fighting with “Bonnie Prince Charlie” at Culloden on April 16, 1746.
Once the rebellion was suppressed, Mercer fled to America, arriving in Pennsylvania in May 1747 and residing briefly in Philadelphia. He eventually moved to the Cumberland Valley on the Pennsylvania frontier, where he established a farm near the village of Conocoocheague and began a successful medical practice. In 1754, at the start of the French and Indian War, Mercer became a captain in the 1st Pennsylvania Militia Regiment and served as a surgeon at Fort Cumberland, treating many of the wounded from British general Edward Braddock’s disastrous attack on Fort Duquesne on July 9, 1755. In September 1756 Mercer participated in a successful attack upon Kittanning, a major stronghold of the Delaware Indians on the Allegheny River. As French allies, the Delaware tribe had been using Kittanning as a staging ground for raids on British frontier settlements. Amid the melee, Mercer’s right arm was fractured by a musket ball, and during the withdrawal he was separated from his company. He eventually escaped to the frontier settlement of Shirleysburg and in May 1758 was promoted to colonel.
In the autumn of 1758 Mercer and the 1st Pennsylvania joined Brigadier General John Forbes’s methodical campaign across southern Pennsylvania toward the Ohio River’s headwaters. He cooperated in building a series of small forts and supply depots along the new military highway, Forbes Road, and was with the vanguard that encountered Fort Duquesne, at present-day Pittsburgh, on November 23. The French garrison had set fire to the fortification a day earlier, and then retreated. Forbes assigned Mercer the task of dismantling the ruins and, with Colonel Henry Bouquet, supervising the building of Fort Pitt. Because Forbes and Bouquet soon departed for Philadelphia, Mercer became Fort Pitt’s first commandant.
Virginia and the Revolution
By 1761 Mercer had returned to civilian life, relocating to Fredericksburg and opening an apothecary shop and medical practice. He married Isabella Gordon of Fredericksburg and they had five children. Mercer also purchased extensive acreage in both Virginia and western Pennsylvania. About 1768 George Washington’s mother moved to a house in Fredericksburg, vacating Washington’s boyhood home at Ferry Farm. Mercer, who had befriended Washington while serving under Forbes, purchased the plantation and became a regular visitor to Mount Vernon.
With the outbreak of the American Revolution, in April 1775, Mercer publicly endorsed “independency.” Given his Jacobite past, he likely possessed scant regard for the Hanoverian king,. With support from Washington and , the General Assembly appointed Mercer colonel in the 3rd Virginia Regiment on February 3, 1775. A little more than a year later, on June 5, 1776, the Continental Congress appointed him brigadier general and placed him in command of a force guarding the northern New Jersey coast from British raiding parties based in Long Island, New York.
In October 1776, Mercer commanded rear guard troops covering the Continental army’s retreat from New York City into New Jersey. He also ensured that British troops did not capture the various Delaware River bridges into Pennsylvania. Meanwhile, in December, Mercer worked closely with Washington on devising a surprise attack upon Trenton, New Jersey, which contained a garrison of Hessian mercenary troops. At dawn, on December 26, Mercer was among the first American attackers to enter Trenton
Mercer devised a comparable plan for nearby Princeton. On January 3, 1777, he led his troops to secure Stony Brook Bridge, subsequently protecting Washington’s left flank as the Continental troops advanced into Princeton. During the British counterattack, Mercer’s horse was shot out from under him and he was clubbed in the head with a musket butt. Badly dazed, Mercer fought back with his sword but still received seven serious bayonet wounds before being rescued by his men rescued.
He was carried on a litter to the nearby farm of Thomas Clark. Several army doctors attended to him while Washington arranged for the eminent Philadelphia surgeon Dr. Benjamin Rush to handle the case. Although there was hope he might survive, Mercer died on January 12, his right arm having become severely infected. Mercer’s body was transported into Philadelphia by a hearse that rode over the thick ice that covered the Delaware River; he was interred at Christ Church Cemetery there. By 1840, his grandchildren arranged to have Mercer’s coffin transferred to Laurel Hill Cemetery, also in Philadelphia.