On September 8, 1862 Bristow was commissioned lieutenant colonel over the 8th Kentucky Cavalry. Bristow assumed command of the 8th Kentucky Cavalry in January 1863 after Col. James M. Shackleford, the previous commander, was promoted brigadier general. On April 1, 1863 Bristow was promoted to colonel and continued his command over the 8th Kentucky Cavalry. In July 1863 Col. Bristow and the Kentucky 8th Cavalry assisted in the capture of John Hunt Morgan during his July 1863 raid through Indiana and Ohio.