Allmond knocked out incumbent PHD and Initiative in the final contest to take on the media-planning and buying and strategy for all Weetabix brands, including Ready Brek, Weetos, Mini Crunch and Weetabix cereal bars.
The win follows a change of ownership at the agency, after its management staged a buyout from Omnicom.
Nigel Allmond, chief executive of The Allmond Partnership, said: "This is a fantastic win for us. It represents everything that we have worked hard for this year following our decision to go independent and the investment we have made in expanding our team and range of services. We are privileged to have the opportunity to provide the Weetabix brands with the very best service and performance and are extremely proud to be working on such famous brands."
The review was called in September, just before Weetabix was sold by the George family, its owner of 70 years, to private investment firm Hicks, Muse, Tate & Furst for £642m.
Hicks Muse owns a portfolio of British food brands such as Typhoo Tea, Branston Pickle, Hartley's Jam and Sun Pat peanut butter.
Weetabix has a 13.6% share of the breakfast cereal market, behind the market leader Kellogg.
Weetabix joins BT, which accounts for around 50% of TAP's business, the AA, COI Communications and Gucci on TAP's client list.
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