Both Speed and Mongoose are owned by The Mission Marketing Group, which also owns the PR firm Bray Leino, as well as agencies April Six, bigdog and Story.
Speed Sport's 15 staff will join the existing 15 employees of Mongoose, and a further seven back office staff currently shared by the two agencies, in the new agency on 1 January.
It will operate under the Mongoose brand, and continue to be led by CEO Chris O'Donoghue, who has led Mongoose since it was founded 18 months ago.
Clients of Speed Sport include The North Face, Lucozade, Müller and Bosch, while Mongoose also works with Müller, as well as Green King and Cirque du Soleil.
Mongoose has offices in Singapore and Cork in the Republic of Ireland, in addition to its London HQ, and in April acquired London counterpart Generate Sponsorship.
Kate Bosomworth, who led Speed Sport and is one of three managing partners working under CEO Roberta Fuke, will leave her role, but continue as a consultant to the board in 2017.
Bosomworth, who is also a member of the Sport England board, said: "There are some very exciting things happening in sport right now; I am keen to explore new horizons and take on a new challenge and wish Chris and the team all the very best for the next phase in what I am certain will be a very bright future for all."
Speed boss Fuke said: "It makes sense to combine mission’s specialist sports talents under one brand, leaving Speed Communications to focus on driving its core B2B and B2C business."