The agency won the business after a competitive pitch against the Guinness roster agency RMG Target, which is based in Ireland.
Guinness is looking to ramp up its relationship marketing activity with an emphasis on digital marketing.
The win marks an extension of TMW's relationship with Diageo, which started in 2002. The agency has handled direct work for Guinness in addition to work on Gordon's gin.
The pitch was part of Diageo GB's current review of its below-the-line arrangements across its portfolio of brands, which also includes Smirnoff, Baileys and Pimm's.
Last month, Diageo consolidated its global digital advertising for Baileys into Chemistry to provide a greater emphasis on CRM. The account was previously split between Chemistry and Agency Republic.
In June, Diageo appointed Bartle Bogle Hegarty to handle Baileys' creative account. The agency won the business after a pitch against Abbott Mead Vickers BBDO, Mother and Saatchi & Saatchi.