William H. Powell was promoted, from civilian life, to second lieutenant and appointed to the Fourth Infantry in
October of 1861. On duty at Governor's Island, New York, until December of 1861, he was made acting Adjutant
of the Regiment in February of of 1862 and made Adjutant in March of 1862. Powell was engaged in the siege of
Yorktown and the battle of Gaines' Mill, Virginia. He was acting Assistant Adjutant General of the First Brigade
of Regulars, Army of the Potomac, in June of 1862. He participated in the battles of Malvern Hill, 2nd Bull Run,
Antietam, and Fredericksburg. In September, 1862, he was promoted to first lieutenant (4th U.S. Infantry) and was
brevet captain for gallant and meritorious services during the battle of Antietam.

During 1863, Powell was engaged in the battle of Chancellorsville and later, during the battle of Gettysburg, served
as Aide-de-Camp to Brigadier General Romeyn Ayres. In September of 1863, he was made acting Quartermaster and
acting Commissary of Subsistence of Fort Wadsworth, New York.

Powell was made acting Assistant Inspector General, New York city and harbor, in January of 1864.  He was made
Aid-de-Camp, First Division, Ninth Army Corps, Army of the Potomac (General Ledlie), in June of 1864.  He
participated at Cold Harbor, in early June, 1864, and, in mid June, in the attack on the Confederate lines protecting
Petersburg. In July of 1864, he participated in the Mine Explosion (the Crater).

Powell was promoted to captain in February of 1865 and participated in the operations terminating with the surrender
of Lee in April of 1865. He was acting Assistant Adjutant General for the City of Richmond, Virginia from April to June
of 1865. In April of 1865, he was also brevet major for gallant and meritorious services in front of Petersburg. He was
mustering officer for Trenton, New Jersey during June and July, 1865 and was Chief of Ordnance and Artillery, New
York city and harbor from August to October of 1865.

He remained with the Regiment after the Rebellion.

William H. Powell was the author of

A History of the Organization and Movements of the Fourth Regiment of Infantry

United States Army, From May 30, 1796 to December 31, 1870

and of

The Fifth Army Corps