Answer: Lyndon B. Johnson, who joined the Navy immediately after the Pearl Harbor attack.
“When the United States entered World War II, Johnson became the first member of Congress to enlist in the armed services, becoming a lieutenant commander in the Navy. His military service abruptly ended, however, when President Roosevelt ordered that members of Congress choose between serving in uniform or in Congress. Johnson resigned his active commission and returned to Capitol Hill.”
Source: United States Senate Historical Office
Lyndon. B. Johnson was born on August 27, 1908.
Eight future presidents served during World War II. The others are: Eisenhower, Kennedy, Nixon, Ford, Reagan and George H.W. Bush. Jimmy Carter was at the Naval Academy during the war, graduating in 1946.
Read more about Johnson’s life and administration in Don’t Know Much About History and Don’t Know Much About the American Presidents.