MI5's earliest antecedent was a secret service formed in 1569 by Sir Francis Walsingham (Walsingham, Sir Francis), who later became secretary of state to Elizabeth I. In the early 20th century it was realized that some form of centralized control of intelligence functions was necessary. MI5 was formed in 1909 under the leadership of Vernon Kell, then a captain in the British army, to identify and counteract German spies then working in Britain, which it did with great effect. Kell retired as a major general in 1924 and was later knighted but remained in charge of the agency until 1940. (The name “MI5” originated during this period, when the agency was “section five” of military intelligence.)

MI5 enjoyed great success during World War II, but its record during the Cold War was mixed. Widely publicized blunders during that period—e.g., the Soviet Union was found to have deeply penetrated both MI5 and MI6, the agency responsible for foreign intelligence—undermined confidence in MI5. A secretive organization, it first publicly named its head in 1991. At that time it also made public some previously classified information, such as the number of its employees and its organizational structure. Counterterrorism operations account for much of MI5's activities. MI5 reports to the Home Office.