He will be responsible for accounts including Deutsche Bank and MVC Entertainment.
His remit will also be to drive the company's growth in London, as well as build the agency's international business.
The agency has lost profile in the UK over the past few years. It is the 98th-biggest advertising agency, with £6.3 million in billings.
Mamier will work closely with the new strategic services director, Benjamin Pagel, and the agency's chief creatives, Trevor Kennedy and Steven Spence, who co-founded the London office in 1998.
Kennedy said: "Our unusual concept of a German-owned agency, composed mostly of UK staff with a Frenchman as MD, will strengthen our competitive edge. As a medium-sized agency with an excellent creative record, we can provide unique pan-European brand expertise."
While at Seven Worldwide, Mamier picked up new business from brands such as Stella Artois, Leffe and Castlemaine XXXX.
Before that, he played a major role in the growth of CNBC Europe as the marketing manager for Europe.