He joined the United States Marine Corps in 1942 and became an officer. In addition to seeing combat with the 4th Marine Division, he played for a service football team in Maui, Hawaii. He served with I Company, 3rd Battalion, 23rd Marines through the battles of Kwajalein, Saipan (earning a Silver Star), and Tinian. On February 19, 1945, 1st Lieutenant Johnson was killed in action by a mortar shell at the Battle of Iwo Jima and awarded a second Silver Star posthumously; he was one of three former NFL players to die on Iwo Jima along with Jack Chevigny and Jack Lummus. Johnson was buried at the National Memorial Cemetery of the Pacific in Honolulu on February 2, 1949.