In 1861, Stoneman entered the Union army as a captain in the 2nd US Cavalry. He was promoted to major, and served on the staff of General McClellan during the West Virginia campaign. Stoneman became chief of cavalry of the Army of the Potomac with the rank of brigadier general in August 1861. In March 1863, he was promoted to major general. Stoneman commanded forces during the Peninsula campaign, the Fredericksburg campaign, and the Chancellorsville campaign. He was replaced as the chief of cavalry after his failed advance toward Richmond in May 1863. Stoneman served in the cavalry bureau in Washington before he was sent to the West in 1864. He was captured shortly after his service in the Atlanta campaign, while trying to free prisoners from Andersonville prison. After the war, Stoneman was brevetted as a major general for his service.