Despite his complete lack of military experience at the outbreak of the Civil War, Wadsworth was commissioned a major general in the New York state militia in May 1861. He served as a civilian volunteer aide-de-camp to Maj. Gen. Irvin McDowell at the First Battle of Bull Run on July 8. McDowell recommended him for command and, on August 9, Wadsworth was commissioned a brigadier general; on October 3 he received command of the 2nd Brigade in McDowell's Division of the Army of the Potomac. He then led the 2nd Brigade, 3rd Division, of the I Corps of the army until March 17.