Regan will leave the Interpublic-owned agency on 30 November after just over a year in his present role.
Prior to his time as chief strategy officer at the agency, he held the position of head of planning and insight and has also held senior roles at creative agency The Red Brick Road, where he was communications director, and also WPP communications planning offshoot Nylon.
Regan's promotion to chief strategy officer was part of an ongoing restructure of the IPG Mediabrands agency following the appointment of Alex Altman as chief executive at the end of 2010.
His remit in the post was to focus on the continuing transformation of Initiative into a performance-led company, bringing together the agency's insight and evaluation specialists, with practitioners in response, digital analytics and economics.
Reagan has a wide client brief across the agency. Initiative’s client roster includes Tesco, Burger King, Hertz and Flight Centre.
A spokeswoman for Initiative said: "We can confirm that Tony Regan is leaving Initiative. We are now assessing our needs in the area of strategy and planning."
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