Emerged after World War II, when Tokyo changed from a city (shi) to a metropolis (to), putting it on the level of a prefecture (ken) without actually being a prefecture or an urban agglomeration (fu) like Kyoto or Osaka.
The Metropolitan Government is now headquartered in west Shinjuku, in a fifty-story building aptly called the Metropolitan Government Building (Tochô). The Tocho is Tokyo's tallest building, and appears in the Beastie Boys' music video for "Intergalactic."
In its spare time, the Metropolitan Government remains an aloof bureaucracy, much like Japan's other institutions of governance.