M is a code-name given to figures in British Intelligence
- M is the man to whom Lord Peter Wimsey reported during his career in espionage in the Second World War, according to "The Wimsey Papers", a series of articles by Dorothy L. Sayers published during that conflict in The Spectator.
- M is the codename of Admiral Sir Miles Messervy, the head of the Secret Intelligence Service (better known as MI6) in the James Bond novels of Ian Fleming. His successor, Barbara Mawdsley, was also known as M.
- In The League of Extraordinary Gentlemen by Alan Moore and Kevin O'Neill, the head of Military Intelligence Group Five - "M" - is revealed to be none other than Professor Moriarty. He is succeeded in his role by Mycroft Holmes.