On August 12, 1861, Patrick H. Monaghan became one of the early responders to U.S. President Abraham Lincoln's call for volunteers to defend Washington, D.C. following the mid-April 1861 fall of Fort Sumter to the Confederate States Army. After enrolling for military service in the American Civil War, Monaghan officially mustered in as a private with Company F of the 48th Pennsylvania Infantry. Military records at the time described him as being an 18-year-old laborer and Schuylkill County, Pennsylvania resident who was 5’7” tall with light hair, blue eyes, and a light complexion.