[2] Once the American Civil War started, Cook sided with the Confederate States of America and enlisted as a private in the 4th Georgia Volunteer Infantry.

By the end of the Seven Days campaign on the Virginia Peninsula, Cook had advanced to the rank of lieutenant colonel.

After recovering, he fought under Maj. Gen. Stephen D. Ramseur at the Battle of Cedar Creek in the Shenandoah Valley before returning with his men to the trenches around Petersburg, Virginia.

After the war ended in early 1865, Cook moved to Americus, Georgia, where he set up a law practice and was active in local and state politics.

From 1873 to 1883, Cook was a member of the United States House of Representatives as a Democrat,[3] serving a district comprising part of southwest Georgia.