Mills was formerly the managing director of FHV/BBDO in The Netherlands.
Mills, who is due to start in his new position based in Moscow at the end of June, will be responsible for ADV's operations across the Commonwealth of Independent States, a confederation of former Soviet republics.
The ADV stable of companies includes McCann Erickson, Lowe, Initiative and Universal McCann.
Mills started his career at Saatchi & Saatchi in London. He joined Abbott Mead Vickers BBDO to run the Pepsi business in Europe and was the managing director of BBDO Portugal and the chief executive of BBDO Moscow before joining FHV/BBDO in 2000.