Pickett graduated last out of 59 cadets in the United States Military Academy class of 1846. He served as a second lieutenant in the United States Army during the Mexican–American War and is noted for his service in the Battle of Chapultepec in September 1847. After this, he served in the Washington Territory and eventually reached the rank of captain. Pickett participated in the Pig War of 1859. Near the beginning of the American Civil War, he enlisted in the Confederate States Army, and he attained the rank of brigadier general in January 1862. He commanded a brigade that saw heavy action during the Peninsula Campaign of 1862. Pickett was wounded at the Battle of Gaines's Mill on June 27.