In the years leading up to the Civil War, Wood was a conductor on the Underground Railroad, helping hundreds of former slaves escape to New England and Canada. Wood led drills in preparation for John Brown's raid on Harpers Ferry, but ultimately objected to crossing state lines under arms and withdrew from the raid. Following the outbreak of the war, Wood was appointed by Secretary of War Edwin Stanton to be superintendent of the military prisons of the District of Columbia, principally the Old Capitol Prison.