More reasonably, J. S. Callaway suggests that Mycroft Holmes was the head of the Secret Intelligence Service of the British government, whose identity was carefully concealed by the government. Billy Wilder’s film The Private Life of Sherlock Holmes (1970) makes the same identification (with Christopher Lee, who played Sherlock in the German film The Deadly Necklace (1962), in the part of Mycroft), and Ian Fleming’s reference to James Bond’s superior as “M” may reflect a hereditary title applied to Mycroft’s successors.
–The New Annotated Sherlock Holmes, Vol. I, by Leslie S. Klinger