Armstrong served as a member of the board of freeholders which framed the charter of St. Louis in 1876, and was appointed as a Democrat to the U.S. Senate to fill the vacancy caused by the death of Lewis V. Bogy, serving from September 29, 1877, to January 26, 1879, when a successor was elected and qualified. Armstrong was not a candidate for reelection in 1879; and in 1893 died in St. Louis. Interment was in Bellefontaine Cemetery.
Armstrong was Vice President of the St. Louis Board and helped command the July 27, 1877 cavalry attack on the strikers outside Schuler Hall, the headquarters of the Executive Committee coordinating the St. Louis General strike. He reportedly yelled from a shady side of the street for his policemen to "Ride 'em down!".[1]
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