John Minnis enlisted at Nicholsville, Kentucky in the 3rd Tennessee Cavalry on November 24, 1862. Appointed first lieutenant, Minnis also took on the responsibilites as the regiment's adjutant. A little over a year later, Minnis received a promotion to major and command of the regiment's second battalion. Minnis's rise through the ranks continued and on July 10, 1864, he became a lieutenant colonel and took over command of the regiment. On September 25, Minnis surrendered a garrison defending the Sulphur Creek Trestle bridge north of Athens, Alabama to Maj. Gen. Nathan Bedford Forrest. Minnis received a parole and he was exchanged on December 16. Upon returning to duty, Minnis became Acting Inspector General of Tennessee Troops. Minnis resigned his commission on March 19, 1865 to take a seat in the Tennessee General Assembly.