The account is valued at around £8m a year and covers paid search, social networking, display and other forms of digital advertising.
MediaCom North is thought to have won out against a number of rivals, in a review process that kicked off more than a year ago.
There was no like-for-like incumbent on the business.
A number of digital agencies previously worked on the business, such as Big Mouth and Oomph.
MediaCom North, based in Manchester, will now buy and plan digital campaigns and advertising across most of the Co-operative businesses, including all its financial services companies, as well as Co-operative Food.
The Co-op is thought to spend around £50m a year in offline advertising.
MediaCom North already oversees the digital planning and buying for other parts of the Co-op's digital business, including travel and funeral care. Other MediaCom North clients include Iceland and JD Sports.