John Addison of Westmoreland County enlisted at Pittsburgh, Pennsylvania on 5 September 1861 as a Private in Co. M, 100th Regiment Pennsylvania Volunteer Infantry. Known as the "Roundhead Regiment" because of the predominance in its ranks of men from Mercer and Lawrence counties whose British ancestors had owed allegiance to Oliver Cromwell, the 100th Pennsylvania saw extensive service from 1861-65, sustaining over 400 deaths on the battlefield and from disease. John Addison would endure more than his share of the latter. Addison was present for duty through June 1862, and participated in his regiment's first campaign on the South Carolina coast at James Island and at the Battle of Secessionville on 16 June. From July 1862 to March 1863, Addison was sick in Washington, DC and West Philadelphia hospitals; thus he was spared the severe engagements his regiment fought at Second Bull Run, Chantilly, South Mountain and Antietam. By early April 1863, Addison was fit enough to join his unit as it maneuvered through Kentucky en route to participate in the siege of Vicksburg, Mississippi. Following the fall of that city on 4 July 1863, the 100th Pennsylvania was ordered to return to the Eastern theatre of war. On this journey many men of the regiment fell victim to an outbreak of congestive malaria; Addison was likely among these because he is listed as "sick in convalescent camp" at Crab Orchard, Kentucky from 10 September 1863 through February 1864 when he rejoined his regiment.

John Addison was present in the ranks for the Overland Campaign of May-June 1864 when the 100th Pennsylvania, as part of the Army of the Potomac's IX Army Corps, engaged in the bloody battles of the Wilderness, Spotsylvania Court House and Cold Harbor. On 29 July 1864 Addison was captured "in front of Petersburg Va. in the line of his duty"; he would spend the next seven months as a prisoner of war at Danville, Virginia. Addison was paroled on 23 February 1865. His original three-year term of enlistment having expired while a prisoner, John Addison was discharged from the army on 6 April 1865. The Civil War ended that same week with Robert E. Lee's surrender at Appomattox. The period image shows IX Army Corps headquarters at Bethel Church, Virginia on 23 May 1864.