Rees will hand over the reins of the agency to Clark in November, though there will be a handover period.
Clark takes on the agency at a time of change. According to last week's IPA income figures, Rees has pushed MindShare's revenue to the top of the agency rankings, though some of the UK's biggest media agencies' income figures are not included in the tables. But despite topping the IPA's rankings, observers suggest MindShare will face difficult issues as it explores ways in which its buying operations will work with its sister WPP shop Mediaedge:cia.
Also MindShare has had a quiet year in terms of new business compared with some of its peers, although its retention of existing clients has been praised as impressive.
Clark said: "One of our most important accomplishments across Asia-Pacific has been building a fantastic team of people. I leave Asia knowing that they will take the company to even greater heights and I am indeed fortunate that Simon has done the same for the UK."
The existing MindShare management team, consisting of the communications director, Rosie Faulkner, the futures director, Jed Glanvill, the investment director, Nick Theakstone, and the finance director, Colin Barlow, will report to Clark.
Rees said he had decided to move on some time ago and had notified Dominic Proctor, MindShare's chief operating officer, of his intentions to leave back in the spring.
"I've always wanted to work for myself. I've made so many contacts with chief executives within the business and I want to use the experiences that I have learned," Rees said.
Rees, who is known for his support of ethical business practices, said that he was in the process of setting up a business that would provide mentoring for companies and their chief executives.
MindShare's former chief, Mandy Pooler, poached Rees from Carat in 1998.
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