William H. Warner was born on June 1, 1837, in New York City to William Warner. His father was a paper agent who owned $200 of personal property by 1860. He grew up and attended school in New York City, and by 1860, he was working as a book binder.
He enlisted in the Union army on June 21, 1861, and mustered in as a sergeant in Company A of the 40th New York Infantry later that day. The regiment took part in the Peninsular Campaign, the Second Battle of Bull Run, the Battle of Antietam, the Battle of Fredericksburg, the Battle of Chancellorsville, the Battle of Gettysburg, and the Overland Campaign. He was promoted to 2nd lieutenant on September 28, 1861, to 1st lieutenant on February 22, 1864, and to captain on July 7, 1864. He was wounded on May 5, 1864, in the Battle of the Wilderness, as a bullet struck him in the left cheek and “emerged at the base of the brain.” He mustered out on June 27, 1865.
Warner returned to New York after the war and worked in the flour milling business along the Hudson River. He applied for a federal pension in July 1866 and eventually secured one. He moved to Ohio in the 1870s, and he was admitted to the National Home for Disabled Volunteer Soldiers in Dayton, Ohio, on March 14, 1879. According to his admission records, he was 5 feet, 6½ inches tall, with brown hair and blue eyes. He married Emma M. Ells on May 6, 1884. They had one child, but he or she died before reaching adulthood. By 1900, they were living in Yellow Springs, Ohio. Warner died there on April 19, 1918.