ZenithOptimedia's UK chief executive, Antony Young, is stepping down.
He will be replaced by Gerry Boyle, the agency's managing director.
Young is moving to an as-yet unspecified job at ZenithOptimedia in the US after three years in charge of the London agency.
Boyle, 34, who joined the then Zenith Media in 2000 as its planning director, was promoted to ZenithOptimedia managing director two years ago. He previously worked at Leo Burnett and Michaelides & Bednash.
Reporting to Steve King, the ZenithOptimedia worldwide chief executive, Boyle will be responsible for ZenithOptimedia UK group companies, including Zed Media and Equinox. Young is expected to move to New York in mid-July, with Boyle assuming his new role on 10 July.
Young, who moved to the UK from Hong Kong, took the reins at ZenithOptimedia shortly after Zenith Media and Optimedia merged.
During his time as the chief executive, Young relaunched ZenithOptimedia as "the ROI agency" and tried to diversify its services beyond core planning and buying. Large account wins included L'Oreal and O2 but, last year, the main agency's billings fell by 12 per cent to £550 million, according to Nielsen Media Research.
Boyle said: "Young is leaving the business in excellent shape. I'm looking forward to the challenges ahead - the world of communications is more fascinating than ever before."