Miles was a carpenter before the Civil War. His Civil War enlistment document reads: "I, Miles J. Barnum, born in Cleveland in the State of Ohio, aged 30 years, and by occupation a Engineer, Do Hereby Acknowledge to have volunteered this 19th day of Aug 1862 to serve as a Soldier in the Army of the United States of America, for the period of Three Years.... Sworn and Subscribed to, at Anna Ills...." His muster roll shows that he was a sergeant in Company "H", 18th Illinois, reduced to private, Company "A", by the Federal Consolidation, and afterward promoted to Hospital Steward. During a forced march from Pinebluff to the Saline River, in Arkansas (Apr 1864) while Miles, then a Sergeant, and his men were trying to remove a wagon from a rut, he was caught between a wagon wheel and a tree and severely injured, suffering a rupture just below the ribs on the left side. He suffered from those injuries for the remainder of his life. Upon the death of his father, Abijah Seth Barnum, in 1869, Miles purchased from his father's estate Lots one and two, Block 84, Plat of Paola, Kansas for the sum of six hundred dollars.