His elder brother John Fenton Mercer (named after an uncle who died and was scalped in the French and Indian War) would also follow their grandfather's and father's career path—admitted to the Virginia bar in Fredericksburg, and operate the family plantation in Spotsylvania County using enslaved labor as well as serve in the Virginia House of Delegates—like their father and uncle John F. Mercer (who also served in the U.S. Congress and as Maryland governor).

Mercer did not travel to Williamsburg for higher studies, but rather to Princeton, New Jersey, after the American Revolutionary War.

The following year Mercer accepted a commission as captain of cavalry from President George Washington, who expected a war with France, which did not happen.

Loudoun County voters elected Mercer as one of their representatives in the Virginia House of Delegates in 1810 and he won re-election multiple times until 1817.

[4] [Jethro Neville of Hardy County, first elected the following year would introduce a similar bill, which also failed to pass.]

[5] In his last term in the House of Delegates, Mercer chaired the finance committee and introduced a bill to construct a canal along the Potomac River.

He also opposed slavery and became active in the American Colonization Society, which he helped found in 1816, Bushrod Washington becoming its president and serving until his death.

[13] Voters in a district encompassing Loudoun and neighboring Fairfax Counties elected Mercer, former President James Monroe, William H. Fitzhugh and Richard H. Henderson to represent them in the Virginia Constitutional Convention of 1829-1830 (with Joshua Osborn replacing Monroe after he resigned).