The agency ran two employee events in Belfast and Dublin for the drinks giant, and a VIP celebration event at the Guinness Storehouse featuring singer Tom Jones.
Diageo may now run the celebrations annually following a successful event it co-hosted with MCI Dublin last Thursday for 250 of Dublin's corporate event buyers at the Guinness Storehouse.
The event was designed to promote the agency, the result of a merger between Ovation Group and MCI in June 2007, and its relationship with the Diageo-owned venue.
MCI director of events Rob Allidine said: "We were honoured to work with Diageo in the spectacular success of the Guinness 250 celebrations. MCI's delivery of three key events centred on the value proposition of bringing people together to celebrate the brand."
Guinness Storehouse managing director Paul Carty said: "We are delighted to partner with MCI Dublin. Together we have undertaken many large events and through our learning and mutual co-operation we have developed an excellent working relationship."