Reporting to Havas Media UK and International chief executive officer Matt Adams, Marks will use her media planning expertise across clients including the BBC, Apple Beats, Kia, PayPal, Axa and Hyundai.
Havas Media UK has been without a managing director since Natasha Murray left in 2016 to join Guardian News & Media.
Marks spent seven years at Maxus, initially as client services director, becoming head of client services before being promoted to deputy managing director in 2014. She began her media career at PHD, where she spent seven years as a media director.
Adams described Marks as a "no-nonsense", "kick-ass" leader: "She is loved by her teams and clients. She will make an immediately positive impact and be a fantastic leader for Havas Media," Adams added.
The appointment comes weeks after Havas won the estimated $100m (£75.8m) Michelin global media account.
Havas has made a number of recent personnel changes. In July, Campaign reported that Havas UK was set to lose five senior media executives; while the same month the group recruited Sarah Treliving as managing director of Arena Media.