John Mercer (February 6, 1704 – October 14, 1768) was a colonial American lawyer, land speculator, and author. Born in Dublin, Kingdom of Ireland, he came to Virginia in 1720 where he built the colonial estate Marlborough (also called Marlboro; at Marlborough Point on the Potomac River, in Stafford County). He was a leading Virginia attorney and lawyer to George Washington, as well as a colonial prosecutor for the King's court of Virginia, particularly in then-vast Orange County.