Admitted to the bar in 1802, Mercer began his private legal practice in
Loudoun County. In 1810 he helped to found the village of
Aldie, Virginia, near his
mill. 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. In his final term, he introduced a bill to establish a system of free public education for white boys and girls, administered by a board of education and financed by the state literary fund. [Jethro Neville of Hardy County, first elected the following year would introduce a similar bill, which also failed to pass.] Mercer served alongside veteran William Noland for several years, then with Thomas Gregg in 1815 and 1816, after which Mercer left for Congress. 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. Meanwhile, during the
War of 1812, Mercer accepted a commission as lieutenant colonel of a Virginia regiment. Promoted to major, he took command of the important defenses at
Norfolk, Virginia. Mercer also served as
inspector general in 1814,
aide-de-camp to Governor
James Barbour and
brigadier general in command of the 2nd Virginia Brigade. Mercer authored a report in the state legislature in 1816 calling for state supported primary schooling for all white children. Supervision was to be provided by a state Board of Public Instruction, chosen by the legislature. In 1817, his bill passed the lower house but died in the state senate. Thomas Jefferson opposed the plan because heavy funding for primary schools would divert money from his beloved state university, and the plan would replace local control by state control. In 1816 Mercer won election to the
United States House of Representatives, then won re-election several times, serving from 1817 to 1839, one of the longest continuous memberships in that era. That seniority helped him become Chairman of the
Committee on Roads and Canals from 1831 to 1839. In 1817, Mercer was elected a member of the
American Philosophical Society in
Philadelphia. At various times Mercer ran as a
Federalist,
Crawford Republican,
Adams Republican,
Anti-Jacksonian and
Whig. Mercer believed in internal improvements and protection of domestic manufactures. Before his service on the House Committee on Roads and Canals, he had been the first president of the
Chesapeake & Ohio Canal Co., serving from 1828 to 1833. 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. By the following year it had a national presence, and worked to establish the Free State of
Liberia.
James Madison,
Henry Clay,
John Marshall,
John Randolph,
John Taylor of Caroline,
William H. Crawford,
Daniel Webster,
Francis Scott Key, and
James Monroe also became founding or early members. Members often debated two troublesome questions: how many free blacks would voluntarily agree to be deported, and who would pay for it? Federal census records are unclear whether Mercer owned slaves in 1820, but he owned no slaves in 1830. Mercer became vice president of the Virginia Colonization Society in 1836, and vice president of the National Society of Agriculture in 1842. In 1853 he again visited Europe, to confer about the abolition of slavery. 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). Fellow delegates selected Mercer to serve on the Committee of the Legislative Department. Increased representation for western Virginia and the gradual abolition of slavery were two key issues at that Virginia Constitutional Convention. ==Death and legacy==