The luxury car brand previously worked with a local digital agency, Energy Source, on digital buying for the last two years.
The account was recently shifted without a pitch. The business will cover all markets in China, across buying of display advertising, social media and search engine marketing.
MEC currently handles overall media planning and buying for Mercedes.
On the account win, Michelle Yang, general manager of MEC China, said: "Given how digital is increasingly playing a more important role in the overall media mix of Mercedes, the client realized that the consolidation of buying and planning within one agency is becoming more important to drive integration."
She added: "The consolidation of digital buying within MEC will enable us to plan and execute campaigns with a more holistic approach across different channels and platforms for Mercedes."
Klaus Maier, president and CEO of Mercedes-Benz China recently said that the automaker's June sales in China could be its best ever.
Sales in the first six months could increase by over 100 per cent from a year earlier. Mercedes-Benz sold 46,100 units from January to May - an increase of 111 per cent year-on-year.