Alexander Elliott became one of the early responders to President Abraham Lincoln's call for volunteers to help defend Washington, D.C. following the fall of Fort Sumter to Confederate States Army troops in mid-April 1861. Following his enrollment in Pittsburgh, Pennsylvania on August 23, 1861, he then officially mustered in at Camp Wilkins in Pittsburgh that same day as a sergeant with Company I of the 1st Pennsylvania Cavalry (also known as the 44th Pennsylvania Volunteers). Military records at the time described him as being a 32-year-old farmer and resident of Beaver County who was 5' 10-1/2" tall with black hair, brown eyes and a fair complexion.