It is the first significant win for the agency since it merged with Geronimo Communications, an integrated agency owned by the Tribal Group, last month.
M&C Saatchi, Delaney Lund Knox Warren & Partners and the Scottish agency Merle competed against Mustoes for the account, in a pitch handled by the AAR.
Mustoes has been briefed to produce a campaign to encourage more people to drink milk, highlighting its health benefits to consumers.
The work will also seek to attract funding for the Milk Marketing Forum, a self-funded organisation, from the European Union. That funding will be used to support dairy farmers' incomes.
Freud Communications, the incumbent on the business, which was appointed in October 2007, was not involved in the pitch.
Despite being briefed at the time to produce a three-year campaign to promote the benefits of milk, the agency never launched any work.
Past advertising for milk has produced a number of memorable campaigns, using the "gotta lotta bottle" strapline, which was devised by the late Rod Allen, a co-founder of Allen Brady & Marsh.
Famous work included the "Accrington Stanley" ad that ran in the 80s, which featured two boys discussing the milk-drinking habits of Ian Rush.