James Thellusson, European MD of Edelman's specialist CSR and crisis arm First&42nd, has been promoted with immediate effect to replace Mahony and become the agency's third CEO in just nine months.
He will have responsibility for running the UK arm of First&42nd and will be assisted by Stuart Smith, who has been promoted from deputy MD to deputy CEO.
International president Alison Canning said that within two years Edelman will launch UK arms of its StrategyOne research business and Blue ad agency, and that Thellusson would then take a group-wide role, leaving Smith in control of the UK PR business.
Mahony announced earlier this week that he is leaving, after less than a year in the post, to set up his own consultancy.
Before taking the CEO role he was joint CEO, working alongside Tari Hibbitt, who stepped down after four years heading the agency.
Canning said: 'Three CEOs in nine months is not great - my expectation is that James will hang around.'
Thellusson is tasked with fulfilling Edelman's pledge of doubling revenue within three years. He is confident there will not be a repeat of the job cuts carried out last year to help achieve this goal.
Mahony leaves the company in September when he is free to trade but is bound by a non-compete clause until December.
His as-yet-unnamed business is set for an October launch and will specialise in reputation management, he said.
He added that in addition to a London office he hopes to take the business to New York and three European markets within five years.
He said he had the backing of two private investors, whom he declined to name, for the venture.
Two directors are understood to have already been hired for the London operation.
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