Called from service in the West, Grover was promoted to brigadier general in April 1862 as he was transferred to the Army of the Potomac. He was brevetted lieutenant colonel in the Regular Army on May 5 for service at Williamsburg, Virginia, and on May 31, colonel, for his service at Fair Oaks. His brigade was often noted for their service with the III Corps at First Manassas. Transferred to the Department of the Gulf, he took command of a division of the 19th Corps until July 1864. He was in command of the right wing of the army besieging Port Hudson, Louisiana. In August 1863, he was promoted major. He commanded a division in the Shenandoah campaign from August to December 1864. Grover was wounded at the Battle of Cedar Creek on October 19, 1864. He was promoted to major general the same day for his service at Winchester and Fisher's Hill. He was mustered out of the volunteer service in August 1865.