The outbreak of the Civil War brought Peck back into the army. He accepted a commission as brigadier general of volunteers on August 9, 1861. He was given command of a brigade defending Chain Bridge and the Northern defenses of Washington at Tennallytown and soon after joined George B. McClellan's Army of the Potomac in Virginia. He commanded the 3rd Brigade, (55th NY, 62d NY, 93d Pa, 98th Pa &amp; 102Pa) Couch's 1st Division, Keyes' IV Corps during the Peninsula Campaign. He served in the siege of Yorktown, and distinguished himself in the battles of Williamsburg and Fair Oaks. He was placed in command of the 2nd Division, IV Corps during the Seven Days Battles where he again distinguished himself. On July 25, 1862 he was promoted to major general of volunteers to rank from July 4, 1862 for his services in the Battle of Malvern Hill.