From August to September, 1861, he helped raise a company of men for military service, and reported to Benton Barracks at St. Louis, Missouri, for induction. He was appointed captain of Company E, Birge's Western Sharpshooters. From December 1861, to February 3, 1862, he participated in the north central Missouri campaign. On February 4, 1862, with the rest of the regiment, Campbell embarked by steamboat, and was transported by river, finally disembarking at Fort Henry, Tennessee. From there, Campbell and regiment marched to Fort Donelson, where the regiment participated in the Battle of Fort Donelson. Later the next month, the regiment fought in the Battle of Shiloh on April 6–7, 1862, although Campbell was not present. The regiment participated in the Corinth Campaign, and served as provost guard in Corinth, Mississippi, after the Confederate commander, Gen. P.G.T. Beauregard evacuated the city on the evening of May 29–30, 1862. In early October, a combined Confederate Army under Maj. Gen. Earl Van Dorn and Maj. Gen. Sterling Price attempted to recapture the city. The Western Sharpshooter fought a bitter skirmish, on October 4, 1862, the second day of Second Battle of Corinth against elements of the Confederate Missouri Brigade in the timber in front of the Federal lines. Col. Thomas W. Sweeny commander of the 52nd Illinois Infantry who observed the fight, declared that the Sharpshooters "fought like heroes". Captain Campbell was not present for the battle. The regiment was transferred to Illinois state control on November 20, 1862. On April 29, 1863, Campbell was promoted to major of the regiment by order of Illinois GovernorRichard Yates for his "gallant conduct" at the battles of Shiloh and Second Corinth. Campbell was not able to join his regiment in many of the battles of the Atlanta Campaign having been transferred to General Sweeny's staff. Captain William S. Boyd carried out field command of the regiment during most of this period. Campbell was present at Decatur, Georgia, on July 22, 1864, where he was wounded. On September 15, 1864, Governor Yates promoted Campbell to lieutenant colonel. He was mustered out with the regiment on July 7, 1865.
Post-war life
Campbell returned to Paris, Illinois, where he died on May 12, 1867.