William S. Collier was promoted to second lieutenant and first lieutenant, Fourth Infantry, United States Army, in
August of 1861. Collier was made acting Regimental Quartermaster and served in that position until April, 1862.
Serving with the 4th Regiment, in 1862 he participated in the siege of Yorktown, and the battles of Gaines' Mill,
Malvern Hill, 2nd Manassas, Antietam, and Fredericksburg. In 1863 he participated in the battle of Chancellorsville,
and later, in July, at Gettysburg. He 1863 Collier also served as the First Brigades Aide-de-Camp (to General Day).
After Gettysburg he was placed on recruiting service and on mustering and disbursement duty at Detroit,

Collier was brevet captain in May, 1863, for gallant and meritorious services at the battle of Chancellorsville. He was
promoted captain in June, 1864. He was brevet major in April of 1865, for gallant and meritorious services during the

Captain Collier remained in the 4th U.S. and was still present with the regiment in the 1870's.