The agency beat Leo Burnett and the incumbent, Bartle Bogle Hegarty. Fallon also competed, but was eliminated at an earlier stage.
JWT will develop campaigns from its London office that will run globally across TV, radio, press and outdoor. It will report to Diageo's head office in Dublin.
The pitch team was led by Alison Burns, the chief executive of JWT London, and Guy Murphy, the worldwide planning director. Creative work on the pitch was directed by Adam Scholes and Hugh Todd. Russell Ramsey, the executive creative director, was not involved owing to contractual agreements with BBH, his previous employer.
The appointment follows that of Agency Republic to the digital creative account, and is the latest in a succession of changes to Diageo's marketing arrangements in recent months.