Antony Young, 38, is a New Zealander and currently the chief executive of Zenith Optimedia Asia. He will take the role in the UK next month.
Simon Marquis, the UK chief executive, moves to the new role of UK chairman to focus on client relationships.
Zenith and Optimedia are currently completing their UK merger, putting the final touches to the management line-up. The European chief executive of Zenith Optimedia, Steve King, has hired Young in the hope that he will bring renewed impetus to the agency after a difficult 2002.
King said: "Based on track record, Antony really is the best chief executive we have got. He's recognised as the strongest leader in Asia and can be a positive catalyst for change. The assets (at Zenith Optimedia) are good people and we need someone with dynamic leadership skills."
Marquis will eventually move to a three- or four-day week. He said: "I'm completely behind this. I've been talking with Steve for several months about how we move forward. I'll have clocked up five years at Zenith in March and am a great believer in refreshing management teams. I want to move on and spend more time on client relationships after spending a lot of time on internal management issues over the last year."
The appointment follows a difficult end to 2002 for Zenith. As it discussed the merger with Optimedia it lost its AA and BT press buying accounts, wiping more than £50 million off its billings.
Young said: "I come in with a completely new perspective and a white sheet of paper. I like to think that everybody is joining a new company and I ask for their commitment and support to build this together. Dealing with people, I'll focus more on their form than their reputation."
Young worked as a media director at Saatchi & Saatchi and then Colenso BBDO in New Zealand before becoming Saatchis' regional media director in the Asia-Pacific region in 1995. He moved to Zenith in 1996.
Greg Turzynski and Tim Greatrex, the joint managing directors of Zenith Optimedia, continue in their roles.