The group's brand portfolio includes Shreddies, Shredded Wheat, Cheerios and Golden Grahams. It controls around 25 per cent of the £1bn UK market.
The consumer side of the brief will focus on building individual brands and promoting the health and nutritional value of Cereal Partners' products. MS&L will also advise on crisis management around issues such as salt and sugar content.
In October, Cereal Partners successfully defended a complaint lodged with the Advertising Standards Authority about a Cheerios TV ad that implied the cereal was healthy. Its defence argued consumers would assume that Cheerios, like most manufactured foods, contained salt and sugar.
The ASA had previously criticised the group for a Shreddies ad that it claimed 'misled' viewers over the cereal's potential to boost children's mental energy.
C&W said it declined to repitch for the business.
Corporate affairs director Andrew Pyne said the brief had been expanded to cover corporate PR.
MS&L director Janice Williams added that Cereal Partners was the agency's first major food client, and it hoped to add more to its portfolio in the future.