On April 18, 1861, at the age of 20, Alex Mitchell became one of Pennsylvania’s earliest responders to President Abraham Lincoln’s call for volunteers to help defend Washington, D.C. following the fall of Fort Sumter, on April 13 to Confederate States Army troops. After enrolling for Civil War military service that day (April 18) at Brookville, Pennsylvania, he then officially mustered in for duty at Harrisburg on April 24 as a private with Company K of the 8th Pennsylvania Infantry. Initially stationed at Camp Slifer, near Chambersburg, Mitchell and his regiment were attached to the 3rd Brigade, 1st Division before being moved, on June 7, to Greencastle and then south to the Washington, D.C. area, where they were assigned to guard duties along the Potomac River near Williamsport. Stationed near Martinsburg in July, the 8th Pennsylvania periodically engaged Confederate troops in small skirmishes, and then participated in the Union Army’s advance on Bunker Hill and in operations near Charlestown later that month. Following the completion of his Three Months’ Service, Mitchell honorably mustered out with his regiment on July 29, 1861.