John Robertson enlisted in the U.S. Army Cavalry at the Philadelphia, Pennsylvania Customs House on October 8, 1942. He attended basic training at the Cavalry Training Center at Fort Riley, Kansas, and specialized training at Fort Bliss in El Paso, Texas. In May of 1943, He was deployed to the Pacific Theater, landing at Brisbane, Australia.
While in Australia, Robertson was transferred to the 8th Engineer Combat Squadron and became a radio operator. He first engaged in combat on the Island of Los Negros in February of 1944. He participated in the invasion of Leyte in the Philippine Islands on October 20, 1944. Robertson remained in the Philippines until August of 1945. He took part in the occupation of Tokyo, where he remained until October 10. He was separated from the Army on November 12, 1945, with the rank of Technical