Mr. Sebastian Mogul is the CEO of the Rainbow Monkey Corporation. He is accompanied by his assistant Simon. In Operation: R.A.I.N.B.O.W.S., Mr. Mogul tricks Rainbow Monkey Fanatic Numbuh 3 by leading her to Rainbow Monkey Island, where real Rainbow Monkeys live, and telling her he intends to give them a residual check. Since Numbuh 3 can smell Rainbow Monkeys he used her to find the real Rainbow Monkeys. He then captured the monkeys and locked them in cages. What he planned to do to the Rainbow Monkeys after that was spoken in a illegible whisper, but it was awful. Mr. Mogul is voiced by Rob Paulsen.
Despite the fact that Mr. Mogul is still owning and running the Rainbow Monkey corporation, he never made another major appearance after Operation: R.A.I.N.B.O.W.S. and the company has shown no further signs of malicious intentions. However he appears in M.O.V.I.E., A.W.A.R.D.S., and Z.E.R.O..
His design and speech pattern seem to be a reference to the late actor W. C. Fields.