The Mushroom King is a seldom-seen ruler of the Mushroom Kingdom, and the father of Princess Peach. Although he is said to rule alongside the princess, he is a figure that is mostly absent in the video game series. However, he does make somewhat notable appearances in other early media such as the Nintendo Comics System and Nintendo Adventure Books, and the character is mentioned on Princess Peach's profile in a 1993 character guide produced by Nintendo of America.
Super Mario Bros.
In the NES instruction booklet of Super Mario Bros., the Mushroom King is briefly mentioned in passing. With few exceptions such as Super Mario Bros. Deluxe, this is generally not included in subsequent releases. This is likely because the Japanese manual does not mention him.
Nintendo Comics System
In the Nintendo Comics System, the Mushroom King is the de facto leader of the Mushroom Kingdom. However, he is depicted as scatterbrained and dimwitted, so his responsible daughter often performs his royal duties in his stead. Wooster, one of his subjects, is occasionally at odds with him due to his intelligence.
Based on his role, it is likely that he was based off the kings from Super Mario Bros. 3 as he plays a similar role in the comic strips (having a magic wand that is often taken by Bowser's forces and being transformed into various animals).
Super Mario Bros. Movie
I'm back... I love those plumbers!”—Mushroom King, Super Mario Bros.
the Mushroom King or The King (as his role is credited) appears as a main character in the Super Mario Bros. film. He is Daisy's father and the king of Dinohattan, but had been overthrown and de-evolved by President Koopa into a giant sentient fungus that had spread across the parallel world to obstruct Koopa's construction projects.
He continually aids Mario and Luigi throughout the film in their quest to defeat Koopa. Once they successfully do so by killing Koopa, the King reverts back to his human form.
He also appears in the webcomic sequel to the film, where he is now called Reznor, his original name from the Super Mario Bros. film script. The name "Reznor" is a reference to the fights scientists believed would have taken place between the T. rex and the Triceratops in prehistoric times, due to the Reznor's similarity to this dinosaur.