Toad has mostly appeared as a NPC throughout all of the games, with his first appearance being in Super Mario Bros., where he was kidnapped along with Princess Peach. In Super Mario Bros. 3, he runs the various Toad Houses in the game, which isn't the first time he has done this (Super Mario 3D Land, New Super Mario Bros. 2, and New Super Mario Bros. U are the other games with Toad Houses run by him).
He appears in Super Mario 64, as the first Toad Mario meets inside of the castle, where he directs Mario to the first world. In Super Mario Sunshine, he plays a minor role, helping out Mario by giving him tips. Super Mario Galaxy and Galaxy 2 places him as the captain of the Toad Brigade. In New Super Mario Bros. Wii, he is seen firing a cannon with powerups at the beginning of the game, then he can be found in the castle, where he offers to show help videos.
Toad makes minor cameos in the Mario Golf series, never having any impact or significance in the games. In the games where he isn't playable in the Mario Tennis series, he is usually found as a referee. In the Mario & Sonic series, he is once again usually found as a referee.
In Super Mario RPG, Toad appears throughout the game as a help to Mario and the party. In the Mario Party series prior to #5, he is either the main host or as someone who hands out items. In Luigi's Mansion, he is found in the foyer of the mansion and sobbing, mentioning that he was sent to find Mario after calming down. In Dark Moon, he and four other Toads are researching ghosts as E. Gadd's assistants.