William Henry Hamner was born on April 22, 1838, in Clarksville, Virginia, to George and Elizabeth Hamner. He enlisted in the United States army in 1856, and he remained loyal to the Union during the Civil War. He was part of the federal garrison at Fort Sumter during the secession crisis, and he was reportedly the soldier who lowered the American flag after the fort surrendered. Decades later, he recalled, “I never will forget the ignomy [sic] of the situation as I pulled down the colors, with all the boats on the river and crowds on the shore whistling and shouting exultation.”
In October 1861, he received a commission as a 2nd lieutenant in Company G of the 3rd Rhode Island Heavy Artillery. The regiment took part in the siege of Charleston Harbor. He was promoted to 1st lieutenant in May 1863, and he eventually earned a promotion to captain. He mustered out on August 4, 1865.
He remained in the Regular Army after the war, and he retired on January 1, 1900. He lived in Los Angeles, California, after retirement. He married Caroline T. Eldridge on June 12, 1919. He died in San Francisco, California, on February 15, 1925.