It is believed Mills departed Admedia on Monday this week, following differences over the future direction of the business.
Mills has been the managing director at Admedia since May 2009, during which time the company has expanded its portfolio with contract wins such as the motorway service station company Extra Services and washrooms at Network Rail stations.
A former sales director of Carlton Screen Advertising, Mills left the cinema sales house in June 2008 after 14 years at the company ahead of its sale to Odeon Cinemas and Cineworld Group and its subsequent rebrand as Digital Cinema Media.
Before joining CSA in 1994, Mills worked in media planning and buying at Abbott Mead Vickers BBDO and TMD Carat.
Philip Vecht, Admedia's chief executive, said: "Adam has been instrumental in building a successful sales and management team and we appreciate his efforts on behalf of the company over the past two-and-a-half years. We wish Adam every success in his future career."
Admedia specialises in outdoor advertising in washrooms in bars and clubs, motorway service stations, shopping centres and rail stations, but also has an extensive network of six-sheets in motorway service stations.