William was born in St Mary’s County, Maryland, and while his birth date and month are unknown, some sources have his birth year listed as 1840, 1841, and 1845. What is known about his life prior to the American Civil War is that he was a farmer that enlisted in the Union Army on the 11th of February 1864, in Norfolk, Virginia. Seven months later, William was a Private with Company C of the newly formed 38th United States Colored Infantry Regiment during the Battle of Chaffin’s Farm, where he displayed actions that later earned him the Medal of Honor. The citation reads:

Among the first to enter the enemy's works, although wounded.

During this battle, the Confederate troops killed, captured, or wounded over fifty percent of the division of black troops and William was one of six to receive the Medal of Honor on the 6th of April 1865, for actions during this battle. He was promoted to Sergeant three months later and he remained in the Army after the war. While traveling to Texas with his regiment, he fell ill of tuberculosis and on the 24th of December 1866, William Henry Barnes died at an Army hospital in Indianola, Texas. He was initially buried there and was later reinterred with other soldiers in a common grave in the San Antonio National Cemetery: Section MA, Site 86.