The pitch - which took place last month (PRWeek, 14 March) - saw the agency battling it out with Cartmell Communications, Performance PR and incumbent Cunning Stunts Communications for the contract.
Mini media relations manager Natalie Wakefield confirmed the appointment of Leeds-based Ptarmigan, adding that Cunning Stunts ceased to work on the account last month.
Wakefield said Ptarmigan would handle consumer projects targeting media, including the lifestyle and regional press.
Upcoming launches include the Mini One D, which goes on sale in the UK in two months' time. BMW is also launching the 'John Cooper Works' kits for Mini's Cooper and Cooper S marques.
Ptarmigan MD Gordon Forbes heads the account, reporting to Wakefield and media relations executive Ellysia Graymore, who are the only PR staff in BMW's 13-strong in-house PR team dedicated to the Mini brand.