During the Civil War, he served first with the 20th Illinois Volunteers. While stationed at Cape Girardeau, Missouri, he recruited an artillery company that became Battery ‘F’ of the 2nd Illinois Light Artillery, with Powell as captain. On November 28, 1861, Powell took a brief leave to marry the former Emma Dean. At the Battle of Shiloh, he lost most of his right arm when struck by a Minié ball while in the process of giving the order to fire. The raw nerve endings in his arm would continue to cause him pain for the rest of his life.