 The youngest son of James Mercer and Eleanor Mercer, Charles Mercer was the first cousin of Robert Selden Garnett and James Mercer Garnett, both also members of Congress.Born in Fredericksburg, Virginia, Mercer graduated from Princeton College in 1797, where he later took a postgraduate course and received his degree in 1800.He studied law and was admitted to the bar in 1802, commencing practice in Aldie, Virginia, a village that he founded in 1810, centering on Mercer's mill.