The appointment followed a three-way pitch (PRWeek, 21 March), understood to have included Shine Communiations and McCann-Erickson PR.
The account win comes after a bad year for British Racing, which was hit by a Panorama documentary alleging corruption within the sport.
Since then the BHB's new director of marketing Chris John has carried out a marketing and comms review and plans to announce a change of strategy in four months' time.
GEM London president Giles Morgan said the main challenge would be to recruit new race-goers and persuade occasional race-goers to attend more often, while maintaining existing support.
Morgan said while it was inevitable that GEM and the BHB had considered the documentary's impact on perceptions of horse-racing, the brief would concentrate on the positive side of the sport.