The account moves from Publicis, which had held the account for the past 15 years, but did not repitch for the business.
Other agencies involved in the Observatory-managed process were CHI & Partners and BETC London, while Abbott Mead Vickers BBDO withdrew at an earlier stage due to conflict.
Sarah Heynen, the marketing director, sweet biscuits, at United Biscuits, said: "More than ever, FMCG brands need real breakthrough creativity to thrive. Grey’s energy, thinking and ideas made them the ideal partner for us."
Chris Hirst, the chief executive of Grey London, added: "McVitie’s are a great and iconic family of brands that deserve great and iconic advertising. We’re excited and a little bit scared to get started."
Publicis will continue to work on the United Biscuits savoury business. The media account handled by MEC is not affected.