Hall volunteered for the United States Navy when World War II broke out, serving as a machinist in Guam. During the first years of the war in Europe, the CPUSA held an isolationist stance, as the Soviet Union and Nazi Germany were cooperating based on the Molotov–Ribbentrop Pact. When Hitler broke the treaty by invading the USSR in June 1941, the CPUSA began to officially support the war effort. During his naval service, Hall was elected in absence to the National Committee of the CPUSA. He was honorably discharged from the Navy on March 6, 1946.