Michael Warner III was born into a wealthy Baltimore family in 1829. His father, Michael Warner, Jr., was President of the Mechanics Bank of Baltimore from ca.1860 to at least 1866, and served as a representative in the Maryland House of Representatives in 1861. Unfortunately, little is known about Michael Warner III's life during the pre-war period: he was one of five children born to his father's first marriage, and he had seven half-siblings by his father's second marriage. Like his father, he appears to have received a formal collegiate education.
In August, 1863, Michael Warner III traveled to Boston and enlisted in Company B of the 16th Massachusetts Infantry. He and the other recruits were ordered to northern Virginia to augment regiments that had suffered tremendous losses in the summer campaigns: Co. B, Warner noted, had only four effectives after being 'very much cut up.' To add to his worries, Warner was robbed of all his money on the way south, a crime he suspected was perpetrated by his fellow soldiers, whom he considered to be "all hard cases & graceless scamps...any quantity of New York pickpockets & blacklegs, murderers & thieves amongst them."
After a year of comparatively uneventful service, Warner was wounded in the arm and eye at the Battle of Spotsylvania, 18 May 1864, and was left on the battlefield as his comrades were driven from the field by Confederate troops. Captured almost immediately, Warner was placed under the care of a surgeon who was forced to amputate his right arm. When the 16th regrouped, and his body was unaccounted for, Warner was listed as missing in action, setting off a several month effort by his father and brother-in-law, John Derr, to try to ascertain exactly what had transpired at Spotsylvania. Some 'witnesses' to the engagement claimed that Warner had been killed, others that he was merely wounded, and one thought he might have been totally unharmed and merely separated from his unit. In the ensuing confusion, Derr and Warner's father contacted hospitals and relief organizations, and attempted to pull strings with officials, but without success: Warner was pronounced dead in September, 1864. In reality, Warner was still alive at that time, and was held as a prisoner in some of the 'more extreme' Southern prison camps. He was paroled and returned to Annapolis in November, 1864, however his fragile health failed him, and he died of chronic diarrhea on December 4.