King Toadstool, usually referred to as the King of Mushroomland, is, as his name suggests, the ruler of the Mushroom Kingdom, and the father of Princess Peach. His character was mentioned in the packaging and instruction manual for the original Super Mario Bros. game, but he is never seen in the games themselves.

However, the Super Mario Bros. comic books published by Valiant Comics did feature a character named King Toadstool. Just as described in the original game's instruction manual, he is the ruler of the Mushroom Kingdom and Peach's father. His design seems to be based off of the King of Grassland from Super Mario Bros. 3. He seems to confide in Mario, and often assigns him to do some tasks, such as gathering Coins with which to balance the royal budget.

King Toadstool is very dim-witted, and as such can easily be duped by Bowser, although he does know that Bowser is his enemy. He usually winds up doing dumb stuff such as closing the peanut butter jar on his tie, and tracking mud on the castle floors. He also believes certain expressions literally, such as the idea that "dark days" and "feeling brighter" relate to lighting bills . In fact, he is so dumb that the explosion of a Stupid Bomb does not have any effect on him. As a result of this, his advisor, Wooster, has written a list of people who say that he is dumb.

King Toadstool also has an eating problem, as shown by the fact that he is overweight. When he goes on a diet, he is known to order some cakes disguised as his crown. His crown looks a lot like the mushroom-like hats worn by the Mushroom People, but seeing how the King is human, his crown may not count as a mushroom hat.

In the Mario comic continuity, King Toadstool is sometimes transformed into an animal by Bowser. He also seems to be a favorite kidnapping victim of Wart, although this was only shown to happen in one story.

King Toadstool also appeared regularly in the Nintendo Adventure Books, where he was very much like he was in the comics.

Several coloring books based on the original Super Mario Bros. game also included a King Toadstool character. However, this King Toadstool looks completely different than the character featured in the comics.

Ironically, the character of King Toadstool (at least the version featured in the comics) seems to be almost a forerunner of the King of Town from the Homestar Runner cartoons.

List of people who say King Toadstool is dumb

• Mario
• Luigi
• Nestor (perhaps a misspelling of Nintendo Power mascot Nester?)
• Kevin Keene
• [Z]elda
• A. Beezo
• M[other] Brain
• D. Owsen
• Wart
• Little Mac
• Donkey Kong
• Dirk Drain-Head