He was a veteran of the Mexican War and the US Civil War and served as the Colonel of the 8th Kentucky Volunteer Cavalry Regiment (US) (1862-63). In Sep/Oct 1862, he was wounded at the Battle of Geiger's Lake, while his 1st Battalion under Major Holloway was engaged with Adam Johnson's rebel cavalry (Morgan's men). He was promoted to Brigadier General in January 1863 and commanded larger cavalry units to include the 24th Cavalry Corps which participated in the capture of Gen. John Hunt Morgan (CSA) at Buffington Island, Ohio in July 1863. Shackelford as the senior Union officer on site claimed credit for the capture.