Thomas enlisted on March 1, 1862 and was sworn in as 2nd Lt., Co. B. 3rd Missouri Cavalry Battalion (Dismounted) at Memphis, Tennessee on April 25, 1862. He had previously served as Aide De Camp on the staff of his father, Brig. Gen. Martin E. Green who commanded a brigade of infantry at the Battle and Siege of Vicksburg, Mississippi where he was killed by an enemy sniper on June 27, 1863.
Company B. 3rd Missouri Cavalry (Dismounted)