William Stewart was born around 1835. By the early 1860s, he was living in Butler County, Pennsylvania. In the summer of 1861, he received a commission as a captain in Company D of the 11th Pennsylvania Reserve Infantry. The regiment took part in the Seven Days Battles, the Second Battle of Manassas, the Battle of Antietam, and the Battle of Fredericksburg. He was wounded at Second Manassas in August 1862, but he eventually recovered and rejoined the regiment. He died at Fredericksburg on December 13, 1862.