The beginning of the American Civil War interrupted Corson's education as he enlisted in Company K of the 4th Pennsylvania Infantry in April 1861, and mustered out as a sergeant in July 1861. Returning to Penn late that year, he served as a medical cadet at the army hospital on Cherry and Brad Streets while studying at the school. He enlisted in the 6th Pennsylvania Reserves (35th Pennsylvania Volunteers) as an assistant surgeon in March 1863 after graduating with an MD degree, participating in the Battles of Gettysburg, Falling Waters, Manassas Gap, Bristoe Station, Mine Run, Rappahannock Station, the Wilderness, Spotsylvania, North Anna, and Bethesda Church. Brevetted major for distinguishing himself in the Wilderness, Corson mustered out again in June 1864. At Bristoe Station on 14 October, Corson and a hospital attendant rescued a severely wounded soldier left between the lines while under Confederate artillery fire. For this action, he was awarded the Medal of Honor on 13 May 1899. In November 1864, he became medical officer at Camp Discharge, used to muster out soldiers, in Philadelphia.