Pierson joined the United States Army and quickly rose through the ranks, meeting Joseph Turner in 1939, during the beginning of World War II. Pierson participated in the Battle of Kasserine Pass in February 1943 alongside Turner, Arthur Crowley, and Davis. When he was ordered to retreat, he refused due to some of his men being trapped at the pass. Pierson ordered his men to hold their ground until reinforcements arrived, but no one came, resulting in a slaughter of many of his men.