John F. Warner was born around 1837. By the early 1860s, he was working as a laborer in Louisville, Kentucky. He married a woman named Julia. He enlisted in the Union army on September 29, 1864, as a substitute, and he mustered in as a private in Company G of the 26th Kentucky Infantry later that day. According to his service records, he was 5 feet, 7 inches tall, with dark hair and hazel eyes. In April 1865, he was promoted to 1st lieutenant in the 125th USCT Infantry.
The regiment was stationed in Louisville, Kentucky, until November 1866, and the men spent the following month on garrison duty at Camp Chase, Ohio. The men were stationed in Cairo, Illinois, until April 1866, when the army transferred them to Fort Leavenworth, Kansas. They moved to Fort Selden in the New Mexico Territory in August 1866.
According to one report, Warner suspected that Lieutenant Frederick Hazelhurst was having an affair with his wife. Julia had accompanied the men to Fort Selden, but Warner forced her to return to the East, and he reportedly filed for divorce soon afterward. Warner confronted Hazelhurst shortly after midnight on October 23, 1866, shooting him in the stomach. Hazelhurst reportedly “ran into the quarters of the commanding officer, but was followed by Warner, with pistol cocked, and bent upon fully accomplishing his bloody designs. Seeing there was no chance of escape, Hazelhurst turned at bay, closed with Warner, and wrenching the pistol from him, shot him dead on the spot.”