In 1942, when he was eighteen years old, Jackson enlisted in the Marines during World War II and became a sketch artist for the Fifth Amphibious Corps. He was wounded during the Battle of Tarawa in November 1943, and later in Saipan. The injuries earned him two Purple Hearts and left Jackson suffering from epileptic seizures, mood disorders, and posttraumatic stress disorder for the rest of his life. In September 1944, Jackson returned to the United States and became the youngest ever Marine Corps combat artist at age twenty. He was stationed in Los Angeles and, in this role, drew and painted "[his] bloodiest close-combat experiences". Jackson was honorably discharged in October 1945. He spent the winter season at Pitchfork Ranch before relocating to New York in May 1946.