The agency was awarded the business without a pitch, and will now be responsible for creating a TV and online campaign, due to break later this year, that promotes the overall Which? brand.
Which? offers consumers product reviews and independent advice to help them make more considered shopping decisions.
The company provides its services through products, including magazines such as Which? Car, Which? Gardening and Which? Money, as well as books, such as The Good Food Guide 2010, iPhone apps and online portals.
Which? has not used an advertising agency in recent years. However, it has previously worked with Watson Phillips Norman, which was appointed to handle the brand's advertising in 2005.
Before WPN, the company worked with Rapier, which helped rebrand the company to Which? from the Consumers' Association in 2004.
Chris Gardener, the Which? commercial director, said: "MCBD stood out because of its creative approach and innovative ideas."