According to the Banjo-Kazooie instruction booklet, Mumbo Jumbo was originally the teacher of the game's antagonist Gruntilda. Mumbo was later betrayed by Gruntilda and was unable to prevent his ex-student from using magic. He was cursed with his mask-like face by Gruntilda until the time of her defeat.
In the first game, Mumbo aides his companions Banjo and Kazooie by offering his magic abilities. Throughout the different game worlds Mumbo has ‘holiday homes’ which Banjo and Kazooie can visit at any time. The shaman however does not give his magic abilities away for free meaning that Banjo and Kazooie must hunt down Mumbo’s missing Mumbo tokens, which make the famous "Eekum Bokum" sound when retrieved.
Once Banjo and Kazooie have acquired these tokens and delivered them to Mumbo at his hut, Mumbo offers his assistance using a transformation spell to turn the pair into some form of native animal associated with the particular game world. Once the pair has been transformed they possess new abilities that allow them to access previously blocked routes. It should be noted that Mumbo does not appear in every level of the game.
In Banjo-Tooie, Mumbo’s original role of transforming Banjo and Kazooie has been assigned to a new character in the series; Humba Wumba. Having spent the last two years after the events of Banjo-Kazooie at witch doctor school, Mumbo now possess more powerful magic.
The shaman takes on a bigger role in this game as he is now a playable character. Once again Mumbo has huts situated in each game world and in order to use his powers, rather than collecting the tiresome Mumbo tokens, the player must acquire a single Glowbo, two of which are hidden in each level (the other is for Humba). Once Mumbo has been given this magic creature the player now has the ability to take control of Mumbo and move around the game world the same way in which Banjo and Kazooie would. Mumbo is by no means as agile as Banjo and Kazooie are as he only has a fairly small jumping height and a single attack where he can zap enemies with his staff.
When the player is in control of Mumbo, their key goal is to find Mumbo pads on the ground. When Mumbo goes to one of these pads he uses his new found abilities to change some aspect of the game world using a spell, such as oxygenating water in Jolly Roger's Lagoon or turning on theme park amusement rides at Witchyworld. This then allows Banjo and Kazooie to access new areas of the game world and collect what would have been blocked Jiggies.
Banjo-Kazooie: Nuts & Bolts (Xbox 360)
Mumbo's main role in the recent third game in the series is as the mechanic who teaches Banjo how to create vehicles. The Workshop itself is located in his new garage in the Showdown Town hub world, which bears more than a passing resemblance to the huts of old, and the classic Mumbo hut music plays in the background during vehicle creation. To unlock new parts for vehicles, players must take the boxes scattered around Showdown Town to Mumbo at his driveway infront of the garage, where he opens them up and can tell the player details of each part's use.
He also features, like most other supporting characters in the game, in LOG's levels as an actor. For instance, in Nutty Acres, he becomes the farmer who is married to Humba Wumba and is in charge of Klungo, who plays the farmhand.