The company called a review in November last year and the pitch process was handled by AAR.
Leo Burnett beat WCRS in a final shoot-out after a four-way pitch that included MullenLowe London and Ogilvy & Mather London.
Leo Burnett first won the account in 2012, when the business was reported to be worth £50 million. It fought off competition from Now and CHI & Partners in a process also handled by AAR. TBWA\Manchester previously worked on the account.
The Publicis Groupe agency also handles The Co-operative Bank, which was not part of the review.
Rocket holds The Co-operative Group’s media planning and buying account.
Leo Burnett created a Christmas ad for The Co-operative Food last year encouraging kindness to strangers. It showed a young man skidding along an icy street and picking up food for an elderly man.
Michael Bates, the group marketing director at The Co-operative Group, said: "Leo Burnett came through a very demanding process and demonstrated a real understanding and appreciation of our Co-op difference."