St. John Richardson Liddell did not receive extensive military training before the American Civil War, and attended the United States Military Academy for only one year, 1833, before dropping out as a result of low test scores. He moved to Louisiana after his brief experience at West Point, and lived on a plantation his father purchased for him in Catahoula Parish. At the outbreak of the Civil War, Liddell volunteered and received a commission as a staff officer to General William Hardee. He later commanded a brigade at the Battle of Corinth, which earned him a promotion to brigadier general on July 17, 1862. Following his promotion, Liddell commanded troops at the battles of Perryville, Murfreesboro, and Chickamauga. After General Braxton Bragg was relieved of command, Liddell was transferred to the Trans-Mississippi Department, where he participated in the Red River Campaign under General Richard Taylor. Shortly after the Red River Campaign, he was put in command of all troops in Mobile, Alabama, and took part in the defense of the city until the end of the Civil War. On April 9, 1865, Liddell was captured at the Battle of Fort Blakely.
After the war, Liddell returned to his plantation, but was killed on February 14, 1870, by a neighboring plantation owner after an argument.