In this newly created role, in which he started on Monday (4 September), Mills will be working alongside the European sales director, Piers Noller, with a remit to increase international sales revenue.
Mills was formerly managing director of ambient media sales company, Admedia. He left there in February and it is believed he departed due to differences over the future direction of the business.
He is a former sales director of Carlton Screen Advertising and has also held senior roles at Abbot Mead Vickers BBDO and TMD Carat.
Media Mountain is an outdoor media company that offers a variety of communication opportunities in more than 200 ski resorts in the Alps.
Bart Marijinen, partner at Media Mountain, said: "We are very pleased to have been able to lure Adam to Media Mountain. He has the wealth of experience and the drive we are looking for to take our company to the next stage of its growth."
Mills said that he was looking forward to talking with agencies and clients about the "fantastic targeting opportunities" that the company could provide.Follow @nickbatten2