Leih succeeds Mike Walsh, who has held the chairmanship since 1990. Walsh, however, will retain his position as the head of the network's European operation, although insiders expect him to retire at the end of 2005.
Joining the agency in mid-May, Leih, 46, will be responsible for the overall direction of companies in the Ogilvy stable. Paul Jackson, Ogilvy & Mather's chief executive, will report to Leih.
Shelly Lazarus, the O&M Worldwide chairman and chief executive, commented: "Gary's entrepreneurial skills and experience in other markets will give us a versatile and vibrant leader. For us to be strong as a global network, London must be strong."
Under Leih's stewardship, Ogilvy South Africa has enjoyed recent success; PricewaterhouseCoopers ranks it as one of the country's top-10 unlisted companies.
Walsh said: "The fantastic contribution that Gary has made to Ogilvy's success in South Africa, along with the fact that culturally he is 100% Ogilvy, make him an ideal candidate to steer the next phase of the UK Group's development."
Leih said: "I'm grateful to Mike for handing on a legacy of strong businesses in good financial health. The task is about focusing our considerable strengths to make Ogilvy the natural partner for clients who view brands as their most valuable commercial asset."
Ogilvy Group UK employs 1,000 people in the UK across agencies including Ogilvy & Mather, OgilvyOne Worldwide, Ogilvy Interactive and Ogilvy Healthworld.
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