The agency will be tasked with staffing various high-profile events for the brands, including the BMW PGA Championship, Goodwood Festival of Speed and Frieze Art Fair.
It will also deliver on smaller events such as pop-up shops, retail activations and conferences across the UK.
Purity will work alongside BMW and Mini’s other agencies, and the account will be led by its client services director, Mark Dunn, and business director Amanda Wenban.
Paul Ferraiolo, marketing director at BMW UK, said: "Purity have a proven track record in providing staffing solutions for major sporting events, roadshows and product demonstration campaigns.
"They showed a huge amount of energy, understanding and enthusiasm throughout the tender process and we are pleased to have them on board for what will be a very exciting few years for BMW and Mini."
Rob Quinn, managing director at Purity added: "BMW and Mini sponsor a range of events and are linked in with a number of highly prestigious events in the UK. We are delighted to be working with them."
Last year the agency delivered a sampling tour around the UK for confectionary brand Green & Black's Organic, as part of the brand's £2m marketing campaign.
Read Event's profile of what it's like to work at Purity here.
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