While Flynn's departure will come as no surprise, his appointment as chief executive of Rentokil at least will. He had been linked to a move to the top job at Australian media company Fairfax.
Lerwill, who has been a non-executive director of Aegis since 2000, will take over from Flynn in late March.
Lerwill spent 10 years at WPP as finance director from 1986-1996. He subsequently joined Cable & Wireless, where he served in a financial and operational capacity as group finance director before being named chief executive of Cable & Wireless Regional division.
Lord Colin Sharman, chairman of Aegis, said: "Robert has extensive experience of the media world, both internationally and in the UK -- key credentials to his appointment.
"Importantly, as a long-standing non-executive director of Aegis, he also knows the business and its management well, ensuring continuity and further development of the company and its growth programme. Aegis is in a strong position to develop and Robert will bring further momentum to this."
Two weeks ago, the industry was awash with reports suggesting that Australian Flynn was set to return home to take over at John Fairfax Limited, publisher of The Sydney Morning Herald.
Flynn was reported to have been interviewed by the board of Fairfax with a view to him taking over as chief executive and leading Australia's second-largest media group.
However, the former newspaper boss at News International appears to have been busy eyeing up a number of future opportunities, as the Sunday Times reported that Rentokil chairman Brian McGowan has been in secret talks with Flynn for three months.
Flynn will be in for a bumpy ride at Rentokil. The FTSE 100 company has been the subject of break-up and takeover speculation recently as its share price fluctuated wildly, jumping 9% last week on rumours. However, the firm, which is valued at £2.8bn is set to put in a poor set of figures in two weeks' time.
The appointment of Lerwill will come as a surprise to the industry. Few names had been mentioned in connection with the top job at the media buying firm as talk of Flynn's exit emerged.
One name that had been thought to be in contention was that of the Aegis finance director Jeremy Hicks.
Flynn leaves Aegis, which owns media agencies Carat and Vizeum, as well as market research business Synovate, after five years, having joined in 1999 from News International where he was managing director. Prior to that, he worked at News Corp in Australia.
Flynn, who was brought up in Newcastle, New South Wales, learned the paper trade on a series of newspapers and also lists 10 years at ICI and a stint as a management consultant on his CV.
Lord Sharman added: "Doug has developed the management team at Aegis, extended our reach in both advertising and market research, and built the business internationally. We wish him well in his new role outside the media sector. There will be a suitable handover period, which will ensure stability within the business."
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