Carnot Posey was born in Wilkinson County, Mississippi on August 5, 1818. He obtained a college education in Jackson, then studied law at the University of Virginia. Returning to Mississippi, he spent some time as a planter and then practiced law in Woodville. Posey served in the Mexican War as a 1st lieutenant in Jefferson Davis's 1st Mississippi Rifles, and was slightly wounded at the battle of Buena Vista. After the War, President
Buchanan appointed him district attorney for the southern district of Mississippi. He held this position until 1861, when he recruited a company called the Wilkinson Rifles. He became colonel of the 16th Mississippi regiment on 4 June, 1861.
Posey fought with the 16th Mississippi at Corinth in June 1861. He subsequently fought at First Manassas, Ball's Bluff, and in all the campaigns of the Army of Northern Virginia. He was wounded during
Thomas J. Jackson's Valley Campaign at the battle of
Cross Keys on June 8, 1862. After commanding Featherston's brigade at Second Manassas and Sharpsburg, Posey was promoted to brigadier general on November 1, 1862, though his appointment was not permanent until January 1863.
His brigade was composed of four Mississippi regiments of infantry, and formed part of Anderson's division of Ambrose P. Hill's corps in the Army of Northern Virginia. He led his brigade at Fredericksburg. At Chancellorsville, Posey's conduct was praised by his superiors. His Mississippians saw limited combat at Gettysburg. At Bristoe Station, October 14, 1863, a minor engagement, but one in which the Confederates suffered heavy losses, Posey was wounded in the left thigh by a shell fragment. Though the wound was not thought to be a mortal one, infection set in. Posey fought the infection for almost a month, but eventually succumbed to it, dying in the home of a friend, Dr. Davis, at Charlottesville, Virginia, on November 13, 1863.
Carnot Posey is buried in the University of Virginia's cemetery.