Publicis, the incumbent on the account for six years, declined to pitch when the review was called in January.
The agency had previously defended the account in February 2006 as part of a statutory review, but despite holding on to the business, it was only awarded a one-year contract.
Stef Calcraft, a partner at Mother, said: "The Post Office is more than a brand, it's an institution. The opportunities it has as an organisation and a business are huge, and we are excited at the prospect of working with a progressive and ambitious client to make this a reality."
Gary Hockey-Morley, the Post Office marketing director, made the appointment.
Hockey-Morley joined the Post Office in September 2005 from Abbey. He replaced Simon Carter, who had left to join Thomas Cook.
The direct account, won by Joshua as part of the statutory review, is unaffected.