On September 24, 1864, Captain William Patterson was mustered out of the 1st Ohio Volunteer Infantry Regiment. This was almost two weeks earlier than the October 7, 1864, date he originally projected. He served with 1st OVI from April 16, 1861, beginning with a three-month enlistment, until September 24, 1864 — a total of three-years and five-months of service.
William returned to the family homestead in Dayton, Ohio. He died on December 4, 1865, of a recurrence of the diseases he contracted during the war.