Alexander (Sandie) Swift Pendleton was born near Alexandria, Virginia on September 28, 1840. He was the only son of William Nelson Pendleton (Episcopal minister, educator, Confederate General) and Anzolette E. Page. The Pendleton family moved to Lexington, Virginia in October 1853, where William became rector at Grace Episcopal Church. Sandie Pendleton graduated from Washington College (now Washington & Lee University) in 1857, and subsequently enrolled at the University of Virginia, where he was pursuing a Master of Arts degree when the Civil War began in April 1861.

He received a commission as 2nd Lieutenant in Provisional Army of Virginia and reported to Harper's Ferry on June 14, 1861. Within weeks, he was asked by General Stonewall Jackson to join his staff as an ordnance officer --- Jackson had known Pendleton from their days together in Lexington, where Jackson was a Professor at the Virginia Military Institute. Pendleton subsequently served as Jackson's Assistant Adjutant General (Second Corps), and the relationship between Pendleton and Jackson was a close one-- it was said that Jackson "loved him like a son." Following Jackson's death at Chancellorsville in May 1863, Pendleton remained as AAG under General Richard S. Ewell and would later serve under Gen. Jubal A. Early. Pendleton married Kate Corbin in December 1863, and the newlyweds were expecting their first child when he was mortally wounded at Fisher's Hill on September 22, 1863. He died on September 23, and in October his body was returned to Lexington for burial. Kate Corbin Pendleton gave birth to a son, Sandie, in November 1864. The child contracted diphtheria and died in September 1865.