After graduating from West Pointin 1844, ranked 11 out of 25 in his class. He served with the infantry in the Mexican War, winning two brevets and suffering a wound at Churubusco. He left the army in 1855 to enter the business world and went to teaching post at his alma mater. Although he did not own slaves, he felt that states had the right to determine whether they would allow slavery or not.
In 1852, he transferred to the commissary branch but resigned 3 years later to engage in the real estate business. In the remaining years before the Civil War, he was adjutant general of the Illinois militia and directed the reorganization of the Kentucky State Guards. As the head of Kentucky's military forces, he attempted to preserve its precarious neutrality. In July 1861, the Unionist-controlled military board of the state ordered the State Guard, which they considered pro-secessionist, to turn in its arms. He supported Kentucky's neutrality, refusing a commission in the Union Army in 1861. Buckner resigned on July 20th, and 2 months later was named a Confederate Brigadier General, neutrality having come to an end.
Initially in command in central Kentucky, he later led a division from there to reinforce Fort Donelson. He directed the attempted breakout from the encircled post on February 15, 1862, but was called back by his superiors, Brigadier Gens. John B. Floyd and Gideon J. Pillow. Both of them fled rather than surrender, and left the task of surrendering the fort to Buckner. He was outraged by Gen. Ulysses S. Grant's demand for unconditional surrender, but he was somewhat mollified by later developments. Buckner was taken prisoner after the surrender.
Exchanged on August 27, 1862, after being held prisoner for 6 months, he was promoted to major general and led his division at Perryville before being ordered to take command along the Gulf Coast. The next spring, he took over the Department of East Tennessee. On July 25, 1863, this command was merged into the Department of Tennessee under Gen. Braxton Bragg but was retained for administrative purposes.
During the buildup prior to the battle of Chickamauga, Buckner reinforced Bragg and his command became a corps for the battle. When President Jefferson Davis visited the army shortly thereafter, Buckner was one of the leading critics of Bragg's generalship. For this reason, Bragg ordered Buckner back off to East Tennessee just before the battle of Chattanooga. There, he served under Longstreet during the siege of Knoxville.
He then held a number of special assignments until again being placed in charge of the Department of East Tennessee in the spring of 1864. During this period, he spent much of his time in Richmond where he became known as "Simon the Poet" for his fondness for writing poetry. Later that spring, he was ordered to the Trans-Mississippi Department as Gen. E. Kirby Smith's Chief of Staff.
After the war, In 1867, Buckner was allowed to return home. He worked successfully in business as a newspaper editor. He wrote and became active in Confederate veterans' groups. After serving as a pallbearer at Grant's funeral, he decided to enter politics. In 1887, he was elected governor of Kentucky as a Democrat, and served for 4 years. In 1896, he ran for the vice presidency on John M. Paimer's Gold Democrats ticket, but lost the election. Buckner died at his home.
At the time of his death, he was the only surviving Confederate officer over the rank of Brigadier General.
Brigadier General - September 14, 1861
Major General - August 16, 1862
Lieutenant General - September 20, 1864
major general and Inspector General, Kentucky State Guard (spring 1860)
Central Geographical Division of Kentucky, Department #2 (September 18 - October 28, 1861)
2nd Division, Central Army of Kentucky, Department #2 (October 28, 1861 - February 11, 1862)
Division, Fort Donelson, Central Army of Kentucky, Department #2 (February 11-16, 1862)
Fort Donelson commander (February 16, 1862)
Division, Left Wing, Army of the Mississippi (ca. September November 20, 1862)
Division, Hardee's Corps, Army of Tennessee (November 20 - December 14, 1862)
District of the Gulf, Department #2 (December 14, 1862 - April 27, 1863)
Department of East Tennessee (May 12-September 1863)
corps, Army of Tennessee (September 1863)
Division, Cheatham's Corps, Army of Tennessee (October - November 1863)
Division, Department of East Tennessee (November 26 -December 1863)
Department of East Tennessee (April 12 - May 2, 1864)
second in command, Trans-Mississippi Department (June - August 4, 1864)
District of West Louisiana, Trans-Mississippi Department (August 4, 1864 - April 19, 1865)
I Corps, Trans-Mississippi Department (September 1864 - May 26, 1865)
Trans-Mississippi Department (April 19-22, 1865)
District of Arkansas and West Louisiana, Trans-Mississippi Department (April 22 - May 26, 1865)