Armstrong served in a number of civil service positions, including postmaster of Paddytown (present-day Keyser);[a] U.S. tax collector appointed by President James Madison; director of the Bank of the Valley of Virginia; member of the Virginia Board of Public Works from 1822 to 1823; justice of the peace for Hampshire County from 1820 to 1852; and Hampshire County sheriff in 1843.
[9] Armstrong served as the second postmaster of the post office in Paddytown (present-day Keyser, West Virginia)[a] from October 15, 1814, until April 20, 1818.
[1][10] President James Madison appointed him a United States tax collector in the 6th District of Virginia in 1813;[9] he also served in this position in 1818 and 1819.
[4][5] In January 1818, Armstrong was unanimously elected as a director of the Bank of the Valley of Virginia's Office of Discount and Deposit in Romney.
[9][14] 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.
[16] In August 1816, Armstrong and John Jack represented Hampshire County at a convention in Staunton to reform the Constitution of Virginia.
[17] Armstrong was finally elected to represent the county in the House of Delegates in 1818, alongside White,[18][19] and served his first term from December 7, 1818, until March 13, 1819.
[5][21][28] Armstrong joined U.S. House representatives Michael C. Sprigg from Maryland, and Andrew Stewart and Chauncey Forward from Pennsylvania to assist in passing a bill to appropriate funds for the construction of the Chesapeake and Ohio Canal in 1828.
[4] The arrival of the Northwestern Turnpike (present-day U.S. Route 50) to Romney in 1830 created a greater demand for inns and hotels in the town.
[38] He relocated to New Creek Station (formerly Paddytown, present-day Keyser) during the American Civil War, where he resided at his son Edward McCarty Armstrong's mansion.