The agency will undertake a major piece of new product development for the brand, which is due to launch later this year.
The overall marketing communications budget for the new product launch is more than £3 million.
Simon Banks, a senior manager for marketing at GSK Oral Care, said: "M&B blew me away with its ideas and its thinking. With this launch, we aim to reconnect people with Aquafresh by actually talking to them and not just the 'dentist' in them. M&B Brand Experiences will be fundamental in hitting this connection."
M&B will be responsible for creating new ways of connecting Aquafresh, one of GSK's top five consumer healthcare brands with an annual turnover of £344 million, with its target market and bringing the brand to life through a variety of brand experiences.
Paul O'Neill, a partner at M&B, said: "Toothpaste marketing hasn't changed much over the years. This client is hell-bent on changing all that. Our brief is to create communications that totally changes the way people view the category."
M&B will work alongside Grey Worldwide, GSK's global advertising agency, and MediaCom, which handles worldwide media planning.
Last September, Diageo appointed the agency, without a competitive pitch, to work on its Scotch whisky brand J&B internationally. The brief was to connect with its core 18- to 24-year-old market in non-traditional ways. Unilever also hired M&B in a similar role to work across a range of its products.