After studying at Phillips Academy, Danville, in Vermont, and at Russell Military Academy in New Haven, Connecticut, he joined the Union Army as captain of the Richmond City Greys—volunteers for service in the American Civil War. Dudley's company was incorporated into the 19th Indiana Volunteer Regiment of the famed Iron Brigade of the Army of the Potomac. At age 19, at the Battle of Antietam, he took command of the regiment after the death of Lieutenant Colonel Alois O. Bachman. Following the battle, Dudley was quickly promoted to Major (September 18, 1862), and then Lieutenant Colonel (October 6, 1862).