He was born in Pittsburgh, Pennsylvania, but was raised by Robert J. Walker, his uncle, who was a Secretary of the Treasury and U.S.

[1][2][3] He attended a private school in Georgetown, D.C.[2] Walker volunteered for U.S. Army service in the Mexican–American War and was appointed first lieutenant in the infantry on February 27, 1847.

[1] Walker served as assistant inspector general, Department of South Carolina, Georgia and East Florida, December 1861 to March 14, 1862.

[1] On May 29, 1862, Walker was in immediate command of a force which drove off a Union Army force at the First Battle of Pocotaligo, South Carolina as they tried to expand their bridgehead from Port Royal, South Carolina, gained at the Battle of Port Royal, on November 7, 1861.

[1][2][3] With the sudden injury of Brigadier General Nathan George Evans, Walker would take command of the "Tramp Brigade" which consisted of the 17th, 18th, 22nd, 23rd, and 26th South Carolina Infantry Regiments, the Holcombe Legion Infantry Regiment and the MacBeth South Carolina Artillery.

On May 20, 1864, William Stephen Walker was severely wounded and captured at the Battle of Ware Bottom Church during the Bermuda Hundred Campaign.

[1][2][3][6] Walker thought he was mortally wounded but was saved by Union surgeon John J. Craven at Fort Monroe, who amputated his leg.