A Zenith employee for 16 years, Greatrex has resigned from the agency five weeks after the arrival of Antony Young as its chief executive. His departure comes ahead of the announcement of a new management structure.
Greatrex said: "This is just what happens in mergers - there is not room for everyone at the top. There were discussions over what I can and can't do, and Antony's got the job I wanted, so I decided it's time to move on and do something else."
Since joining Zenith at its launch in 1987, Greatrex occupied the roles of head of television and deputy managing director before being promoted to managing director 18 months ago. Following the merger with Optimedia, his managing director position was shared with Optimedia's Greg Turzynski.
More changes are expected to ZenithOptimedia's senior management team.
Young said: "Tim has been a good performer and there is a lot of him in the agency. He's young enough to go off and get something else done."
Greatrex refused to comment on his future plans. His recent work at ZenithOptimedia has included launching three new divisions with Zed, Zenith Interactive Solutions and Alliance.