The agency pitched against another, undisclosed agency, to win the business.
It will roll out a national 48-sheet poster campaign to highlight the benefits of eating prunes.
The CPB uses the Sunsweet brand, which commands two-thirds of the world's prune market, to trade prunes on a global basis.
The posters will be supported by press and ambient activity, with possible point-of-sale and promotional work. The overall spend will be up to £600,000.
The pitch was run by CPB's marketing director for Europe, Amanda Bishop.
Mustoes' chief executive, Nick Mustoe, said the agency's creative strategy would reposition the fruit to a younger audience. "Prunes don't have the sexiest image, but we will help change that," he said.