Johnson enlisted in the Union Army at age 15. He was initially enrolled in Company H of the 43rd Ohio Infantry Regiment. The regiment was assigned the command of the XVI Corps in the Fourth Division, but in the spring of 1864, Johnson was transferred to the First Division of the XVII Corps. There, he served under General John W. Sprague. On May 1, 1864, Johnson's regiment joined General William Tecumseh Sherman's army at Chattanooga. The first battle Johnson participated in where he was under fire was the Battle of Decatur. Johnson received a minor leg wound, above the left knee joint, at the Battle of Resaca. Johnson still participated in future battles, such as the Battle of Dallas, the Battle of Kennesaw Mountain, and the Battle of Peachtree Creek. After the fall of Atlanta, Johnson served as an orderly for General Sprague, a position he served in until he was discharged at the end of the war. After the war, Johnson moved back to Lorain County, Ohio.