King Toadstool, usually referred to as the King of the Mushroom Kingdom or the King of Mushroomland, is, as his name suggests, the ruler of the Mushroom Kingdom, and father of Princess Peach. His character was mentioned in the packaging and instruction manual for the original Super Mario Bros., but he is never seen in the games themselves. Adding to the confusion, Nintendo's official website for Mario & Luigi: Partners in Time refer to Toadsworth as Peach's father. This is backed up in the game Mario Superstar Baseball - which would be confusing even without the existence of a previous "Mushroom King", as Toadsworth (an anthropomorphic mushroom) and Peach (a human) are apparently members of two different species.
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 on the King of Grassland from Super Mario Bros. 3. The King confides 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, which includes Nintendo characters such as Mother Brain, Princess Zelda, Donkey Kong, and Little Mac.
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 cakes disguised as his crown. His crown looks a lot like the mushroom-like hats worn by the Mushroom People, but seeing 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 includes 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.
The King made an appearance in the Super Mario Bros. movie as the father of Princess Daisy. In this version, he has been de-evolved into an actual fungus that has managed to spread itself across the kingdom. After Koopa (not referred to as Bowser in this film) is defeated, he inexplicably returns to his humanoid form and delivers a single line ("I love those plumbers"). In his humanoid form he appeared as a gray-haired, bearded king; complete with large fur cape and crown. He is portrayed by Lance Henriksen.
In many games after the movie, he has not been shown a single time. In both appearance and personality King Toadstool is striking similar to the Sultan from Disney's Aladdin. However, this seems to be purely coincidental.