William W. Armstrong, a native of Pennsylvania, joined the 25th Pennsylvania Infantry as a private in May 1861. Mustered out in August 1861, he joined Anderson’s Troop, Pennsylvania Cavalry, an independent cavalry company, on November 30, 1861. The company performed escort and headquarters duty for Generals Don Carlos Buell and William S. Rosecrans, and served at the battles of Perryville, Kentucky and Stones River, Tennessee. He was mustered out of service on March 26, 1863. Armstrong was commissioned a captain in Company H, 65th United States Colored Infantry in February 1864, serving with that regiment until he was transferred to the 92nd United States Colored Infantry in July 1865. Mustered out in December 1865, Armstrong continued to serve in the U. S. Army until 1870.

Armstrong died on February 18, 1915, in Washington, D. C.