Toad first appears as a singular character in Super Mario Bros. 2 as one of the four playable characters. In the game's story, he and his three friends (Mario, Luigi, and Princess Peach) had to help the people of the land of Subcon by defeating the villainous frog (known as Wart) from taking over their kingdom. Among the four characters, Toad was both the strongest and fastest character; however, he had the worst jumps. Toad himself is stated to be the chief guard of the Mushroom Retainers (Toad who were kidnapped in the events of the previous game, Super Mario Bros.). In Super Mario Bros. 3, he begins the story by writing a letter to Princess Peach warning her about Bowser and the Koopalings who were taking over the Mushroom Kingdom and asks her to send help. He also appears in the game by helping Mario and Luigi through running the Toad's Houses (areas that provided power-ups or extra lives for the Mario Bros.). The character then goes on to star in Wario's Woodswhere he prevents Wario from taking over the Mushroom Kingdom with his monsters. Thanks to his own courage and the help of Birdo and the fairy named Wanda, Toad manages to defeat Wario in the end and chase him out of the Woods (thus reversing the antagonist's control over the woods and monsters). For the next few years, Toad appears in many of the Mario spin-off games such as the Mario Kart series Mario Party series (where he is often featured as a playable character) and various other sports games.
Toad appears in the various Mario 3D series games such as Super Mario 64 where he was the first Toad that Mario met in Peach's castle and he explains much of the back story of what had happened to Mario. In Super Mario Sunshine, Toad (as he was named in the instruction booklet) accompanied the princess, Mario and Toadsworth with four other Toads to Isle Delfino as one of her attendants. In the Super Mario Galaxy games, he is the captain of the Toad Brigade (naming himself as so by having a head lamp). As a result, he was named as the Toad Brigade Captain by his fellow Toad Brigade members. Much of Toad's original personality is retained in the Galaxy games (once again being impatient and cowardly yet loyal) and he assists Mario on his adventure throughout the galaxies in this game as well as its sequel. In Super Mario 3D Land, Toad plays the role as Mario's sidekick as he continually assists the hero by providing him with assistance through the red Toad Houses (which Toad himself runs) and the provision of cannons for the airship levels. Toad also appears in a certain few levels where he must be spotted through the binoculars to see the help he would provide (such as tossing down star coins near to Mario or providing free power-ups/1-Up Mushrooms).
Toad also makes a brief appearance in Super Smash Bros. Melee, in which he can be seen running back and forth frantically in the first stage of adventure mode. He is also a collectible trophy. He is also one of Princess Peach's attacks in which she throws Toad in front of herself to take the blow of a foe's attack; this attack of Peach's will be reused in Super Smash Bros. Brawl. There, he will also appear as a sticker. In other games, he acts as an instructor, such as in the earlier Mario Party games, and helps guide characters in other games. As with the games, Toad plays a major supporting character in other forms of media starring Mario. He usually tags along with Mario in the various cartoons and comics as a sidekick.
In the Super Mario Bros. live-action film, Toad (portrayed by Mojo Nixon) was re-imagined as a street musician and one of the poor inhabitants of Dinohattan, all of whom were descended from dinosaurs; unhappy with King Koopa's rule, Toad was placed under arrest by King Koopa's police force for singing "anti-Koopa songs" and placed in custody, where he befriends Mario and Luigi. Toad is later forcibly devolved into a Goomba, but apparently retains his humanity, as evidenced by his kindness towards Princess Daisy and the fact that he aids Mario and Luigi, giving them the devolution gun that is ultimately used to end the threat of King Koopa.
He is well known for the quote "Thank you, Mario, but our princess is in another castle" from Super Mario Bros..