The agency triumphed following a pitch against WCRS and Hurrell and Dawson. The previous incumbent, Propaganda, which split with the company earlier in the year, did not repitch.
The biscuit manufacturer is looking to boost sales in the face of rising costs of ingredients and has also appointed Brandopus to rebrand its packaging.
Steven Hopkins, a strategist at Mother, said: "The Fox's brand is a national institution and we couldn't be happier about working with them. We're also pleased about the prospect of eating biscuits all day in the name of research."
Mother's first work for the brand is expected to break in January.