James Mercer (ca.1735–1793) was the son of John Mercer (1704-1768) of Marlborough in Stafford County, Virginia, a brother of George and John Fenton Mercer, and the half brother of John Francis Mercer. Educated at the College of William and Mary, he served from 1762–76 as a burgess from Hampshire County, Virginia. Mercer was also a prominent Fredericksburg lawyer and a judge of the General Court 1779–89. He later served on the Virginia Court of Appeals from 1789 until his death in 1793. James Mercer was served as the trustee of the estate of his father John Mercer, which John Francis Mercer later managed.

Mercer was married to Eleanor Dick Mercer of Fredericksburg. Thier son, Charles Fenton Mercer (1778–1858), was a graduate of the college at Princeton, and from 1797 to 1802 he read law at Princeton and in Richmond.