Super Mario Bros 3 portrays him as being king of the Dark Land.

Source (from [Wikipedia][1]):

> When the brothers rescue the seventh king, the letter they receive reveals that Bowser has kidnapped Toadstool and is holding her captive in the castle of *his own kingdom, Dark Land.* The brothers travel through Dark Land, enter his castle, and defeat Bowser in a battle.

*Picture of the letter from both the original SMB3 and the All-Stars remake.*

[![enter image description here][2]][2]


  [1]: https://en.wikipedia.org/wiki/Super_Mario_Bros._3#Plot_and_characters
  [2]: https://i.sstatic.net/53HBq.jpg