Bradley enlisted in the Army from Walden in August 1862, advanced to become a sergeant, and later received a commission. After the Battle of Chancellorsville, he was promoted to captain and became the aide-de-camp to Major General Gershom Mott, 3rd Division, II Corps. He later received a brevet promotion to major of Volunteers. Bradley was wounded at the Battle of Gettysburg, the Battle of the Wilderness, and the Battle of Boydton Plank Road, and was mustered out with his regiment in June 1865. After the war Bradley was active in the Military Order of the Loyal Legion of the United States and other veterans organizations and reunion societies. Bradley also continued his military service as assistant inspector general of the New York National Guard with the rank of lieutenant colonel, later receiving promotion to colonel.