Marimbo is the ancient and cheeky matron goddess of monkeys. At some point, she went to Ape Atoll in search of followers. There she found monkeys; she may or may not have granted them their intelligence. Marimbo decided to stay on Ape Atoll, and the monkeys there still worship her to this day. Once the Edicts of Guthix were established, Marimbo was forced to leave Gielinor and, according to Mod Osborne, because Marimbo is a light-hearted character she partied once the Third Age ended and the Edicts were introduced. This contrasts with the other gods, who were not happy to leave.
The town Marim, on the island of Ape Atoll, is named after her and her only known place of worship is the temple there. Large statues of Marimbo sit inside the Temple of Marimbo on Ape Atoll. Players may see them next to the altar on the first floor of the building. The statues depict her as a giant gorilla. Her symbol is a triangle (or diamond) which points downwards, with a vertical spike hanging from the bottom point.
Eventually after the Assassination of Guthix, Marimbo returned to Gielinor, only wanting the whole world to have fun and to party, rather than to constantly war.
During Cabbage Facepunch Bonanza, Marimbo will tell players of her ascension, which she recalls little about. She reveals that she was just an average non-sapient gorilla, and that she 'just one day woke up'. When she became aware, she found herself next to the stony corpse of a dead god along with an empty wine barrel, whose followers nearby then told her about the dead god being hedonistic as well, and travelling worlds challenging mortals to drinking contests. It's suggested that Marimbo defeated him in a drinking contest and that the overdose of alcohol killed him, which might have drained his divine energy into Marimbo in some way.
She also mentions a fancy, sparkling drinking horn she obtained that she had lost, which is likely to be the elder horn, implying she may have lost it in the Eastern Lands.
In a Twitch live stream with Mod Osborne, it was confirmed that the Pest Control Islands once belonged to Marimbo before they were taken over by the pests.
Before the Missing, Presumed Death quest, Marimbo was widely regarded as a male god, and in game sources were contradictory on the subject (although it seemed to favour that Marimbo was male). Sometime before Missing, Presumed Death, Marimbo's gender was decided by Jagex as being female, due to the noticeable lack of female Gods in Gielinor.
Judging by the presence of a podium displaying Marimbo's symbol at the Empyrean Citadel in Missing, Presumed Death, she had received an invitation to Sliske's Ascension. However, Marimbo was more concerned with hosting her own welcome back party, and so did not attend.
The version of Marimbo's symbol seen on the podium in the Empyrean Citadel appears to be a triangle on a stick, as opposed to a diamond.