William Douglas was born in 1841 in Albemarle County, Virginia. He enlisted in the Union army on February 17, 1865, near Wheeling, West Virginia. At the time of his enlistment, the 24-year-old farmer stood 5 feet, 4 inches tall, with black hair, hazel eyes, and a "yellow" skin complexion. Douglas mustered into the army, serving as a private in Company C in the 45th USCT Infantry Regiment, on February 18, 1865, in Wheeling, West Virginia. The 45th USCT found itself in multiple important moments in the war, including the fall of Richmond and the Appomattox Campaign, which culminated in the surrender of Confederate General Robert E. Lee. The regiment was then sent to Brownsville, Texas, in order to patrol the border with Mexico. Douglas and the rest of his unit mustered out of the army on November 4, 1865, near Brownsville.

He may have married Hester Burton around 1862. Her father was Aaron Burton, the enslaved "body servant" of Confederate colonel John S. Mosby. If this was the same William Douglas, then the couple moved to Brooklyn, New York, after the war, and they had a daughter named Anna. He earned a living as a porter, and his wife worked as a laundress. In 1900, his household contained his elderly father-in-law as well as his daughter Anna and her husband. His wife died on March 20, 1904, and he probably died sometime afterward.