The departure follows the takeover of MLIM by the New York-based investment management company BlackRock earlier this year. BlackRock is an existing Ogilvy & Mather client.
The loss has no effect on the wider Merrill Lynch account, held globally by JWT and run out of its New York office.
O&M is understood to be working on an autumn relaunch and rebrand of the company, which will trade under the BlackRock name.
BlackRock and Merrill Lynch announced they had reached an agreement to merge MLIM into BlackRock in February. Merrill Lynch has a 49.8 per cent stake in the new independent investment company.
The combined company, which is expected to start trading in September this year, will offer equity, fixed income, cash management and alternative investment products.
It will have more than 4,500 employees and offices in 18 countries, including the UK. The company predicts that it will have $1 trillion in assets under its management.
The account move and rebrand will call into question MLIM's sponsorship of the ATP Tour of Champions tennis event. MLIM inked a three-year deal in February.