The decision brings to an end Nationwide's long-standing relationship with MEC, which has handled the account for 12 years.
Nationwide said it was especially impressed with MPG's digital offering during the pitch process, which was handled by the agency search business OysterCatchers. The bank now plans to divert much of its media spend online.
The review does not affect Nationwide's relationship with its creative agency, Leagas Delaney.
Peter Gandolfi, the head of brand strategy at Nationwide, said: "MPG demonstrated real passion, inspiration and understanding of the business. We were also impressed with its digital capabilities."