William Thomas Magruder was born in Upper Marlboro, Maryland during the 1820s. He was the son of Fielder and Matilda Magruder.

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. Upon his graduation Magruder was brevetted to second lieutenant and sent to cavalry school at Carlisle, Pennsylvania. On October 9, 1851, Magruder was promoted 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). Magruder was promoted to first lieutenant on March 3, 1855.

As sectional tensions mounted and states began to leave the Union, Magruder was promoted to captain with the 1st Dragoons on January 8, 1861. Soon after the American Civil War began, Magruder commanded a company at the Battle of Bull Run I (July 21, 1861). In 1861, the 1st Dragoons were re-designated as the 1st U.S. Cavalry, and Magruder was appointed as a captain in that regiment on August 3.

Magruder served with the 1st Cavalry during Major General George B. McClellan's Peninsula Campaign from July 1 to August 3, 1862, when he was granted 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.

Upon joining the Confederate Army, Magruder was commissioned 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 was killed in action while trying to rally the men of his brigade during Pickett's Charge at the Battle of Gettysburg. The disposition of his remains is unknown.

William T Magruder