Armstrong first ran for election as a
Democratic-Republican to represent Hampshire County as a member of the
Virginia House of Delegates in 1811, however, he lost to
Federalists Alexander King and
Francis White. In 1812, he ran again for election to represent the county in the House of Delegates losing to King and White. In August 1816, Armstrong and John Jack represented Hampshire County at a convention in
Staunton to reform the
Constitution of Virginia. Armstrong was finally elected to represent the county in the House of Delegates in 1818, alongside White, and served his first term from December 7, 1818, until March 13, 1819. He was reelected with White in 1819 and served his second term from December 6, 1819, until February 25, 1820. Edward McCarty and White succeeded him in 1820. In 1816, when
Virginia's presidential electors convened at his father's hotel in Romney, Armstrong became interested in national politics. and
1824 U.S. presidential elections. Armstrong ran for election as an
Adamsite Democratic-Republican against Federalist
Edward Colston to represent
Virginia's 16th congressional district in the
U.S. House of Representatives in 1825, and won. Virginia's 16th congressional district comprised
Berkeley, Hampshire,
Hardy,
Jefferson, and
Morgan counties in the present-day
Eastern Panhandle of West Virginia. He was elected for a second term as an Adamsite Democratic-Republican and served in the
20th Congress from March 4, 1827, to March 3, 1829. Armstrong was elected for a third term as a National Republican and served in the
21st Congress from March 4, 1829, to March 3, 1831, and for his fourth term as a National Republican and served in the
22nd Congress from March 4, 1831, to March 3, 1833. == Later life and death ==