William was born on the 9th of June, 1866, in Massachusetts and his father died when William was 21 years old. After enlisting in the US Navy, he was deployed in support of the Spanish-American War and was a Machinist First Class when his actions would earn him the Medal of Honor. The citation reads:

On board the U.S.S. Marblehead during the operation of cutting the cable leading from Cienfuegos, Cuba, 11 May 1898. Facing the heavy fire of the enemy, Hart set an example of extraordinary bravery and coolness throughout this action.

William died on the 6th of February, 1899 at the age of 32, just nine months after his actions. His mother, Sarah, received his Medal of Honor on the 7th of July, 1899, ten years before she would die as a result of falling down her cellar stairs at the age of 80. William H Hart is buried with his parents in the Green-Wood Cemetery in Brooklyn, New York: Section 4, Lot 21072, Grave 22.