The Havas-owned agency won through in a pitch involving Aegis-owned Carat, and the incumbent, WPP's Mediaedge:cia, which has handled the account for the last 12 years.
The review, which was called in December, was overseen by Oystercatchers, an agency search business launched in November last year by Suki Thompson, a founder of the Haystack Group, and Peter Cowie, the chief executive of FitchLive and a former head of new-business at JWT.
Peter Gandolfi, head of brand strategy at Nationwide, said: "We have a long-standing relationship with MEC, which we have valued enormously.
"However, clearly, both the media and financial businesses have evolved hugely over the years, and it makes sense to review our systems and processes periodically."
Leagas Delaney handles Nationwide's creative account, which is not affected by the review.