He joined the Naval Reserves in 1940, and then served in the Navy from December 1941 to July 1942. Roosevelt placed him on an observation mission under General Douglas MacArthur in Australia and New Guinea. Johnson went as an observer on an air bombing strike on a Japanese airbase in New Guinea. The Japanese shot down several planes, but Johnson’s plane and its occupants survived. He was awarded a Silver Star. He left the military because President Roosevelt ordered all congressmen to return to their legislative duties.