At the beginning of the Civil War, he went to Augusta and offered his services to the Governor and was given orders to recruit a regiment.".[3] He participated in the First Battle of Manassas under the command of O.O. Howard. For his gallant service at Bull Run he was promoted to brigadier general in March 1862.
Berry was reassigned to the command of the 3rd Brigade of Hamilton's Division (later Kearny's), 3rd Corps. The 3rd Brigade consisted of four regiments: the 2nd Michigan Volunteer Infantry Regiment; 3rd Michigan Volunteer Infantry Regiment, 5th Michigan Volunteer Infantry Regiment and the 37th New York Volunteer Infantry Regiment. Berry's decisive action at the Battle of Williamsburg benefited General Hooker. His brigade fought in the Battle of Seven Pines and the Seven Days Battles around Richmond. Berry was then promoted to Major-General on November 29, 1862.
Berry was placed in command of the 2nd Division of the III Corps, succeeding Major General Daniel Sickles, who had ascended to corps command.
Berry was killed by a sharpshooter's round at 7:26 am (Gould, Edward (1899), Major-General Hiram G. Berry, page 267) on May 3, 1863, during the Battle of Chancellorsville.[4] Next in seniority was Brigadier General Gershom Mott, who was severely wounded; therefore, Revere assumed command of Berry's Division, though Revere's poor decisions led to his court-martial.[4]