During the next fall, Walker treated the casualties of the Battle of Chickamauga in Chattanooga, Tennessee and decided to request commission as an Army doctor again after the battle. In September 1863, Major General George H. Thomas appointed Walker as an assistant surgeon in the Army of the Cumberland, and she was assigned to the 52nd Ohio Regiment. As an assistant surgeon, Walker was the equivalent of a lieutenant of captain and had finally achieved paid work as a surgeon.