William Thomas Magruder was born in Upper Marlboro, Maryland during the 1820s. He was the son of Fielder and Matilda Magruder.
U.S. Military Academy Cadet
Magruder attended the United States Military Academy from July 1, 1846 to July 1, 1850. He graduated eleventh in his class of forty-four cadets.
U.S. Army Officer
Upon his graduation, the army brevetted Magruder to second lieutenant and sent to cavalry school at Carlisle, Pennsylvania. On October 9, 1851, officials promoted Magruder to the full rank of second lieutenant.
Magruder spent nearly the next decade campaigning against American Indians in the West, serving at Fort Snelling, Minnesota (1851-1853), Fort Leavenworth, Kansas (1854), Fort Union, New Mexico (1854-1855), and Fort Tejon, California (1856‑58). Army officials promoted Magruder to first lieutenant on March 3, 1855.
As sectional tensions mounted and states began to leave the Union, officials promoted Magruder to captain with the 1st Dragoons on January 8, 1861. Soon after the American Civil War began, Magruder commanded a company at the First Battle of Bull Run (July 21, 1861). In 1861, the army officials re-designated the 1st Dragoons as the 1st U.S. Cavalry. They appointed Magruder as a captain in that regiment on August 3.
Change of Allegiance – Confederate Officer
Magruder served with the 1st Cavalry during Major General George B. McClellan’s Peninsula Campaign from July 1 to August 3, 1862, when the army granted him a leave of absence. While away on leave, he changed his allegiance and resigned his commission on October 1, 1862 to join the Confederate Army. Magruder was one of four Union officers educated at West Point to switch sides after the Civil War began. The others were Manning M. Kimmel, Richard K. Meade, and Donald C. Stith.
Death During Pickett’s Charge at Gettysburg
Upon joining the Confederate Army, Confederate officials commissioned Magruder as a captain, and he served as a staff officer with Brigadier General Joseph R. Davis’s Brigade, in Major General Henry Heth’s Division of the 3rd Corps of the Army of Northern Virginia. On July 3, 1863, Magruder died on the battlefield while rallying his men during Pickett’s Charge at the Battle of Gettysburg.
The disposition of his remains is unknown.