His role will not be replaced. Hammersley left by mutual agreement with the firm’s owner Vision 7, said EDC, whose brands include Citizen Relations and Elvis Communications.
The changes have also seen the departure of chief talent officer Hasan Khair.
EDC said the company has completed its ongoing operational separation of the UK business from its agencies in North America and the "exploitation of strategic alternatives" continue for its assets in North America.
Hammersley said: "Some while ago we recognised that we needed to return to individual agencies to their entrepreneurial roots. Over the past year or so, we have strengthened each agency’s team, culture and offer and they are all now well positioned and resourced for ongoing success.
"As a result of these changes, the role for the group and a group management team has diminished significantly; now is the right time to move on, particularly in the context of the North American strategic review."
John Bartle, non-executive chairman of EDC UK, said: "Paul has led this process and we would like to thank him for his professionalism and guidance in leading the development of the strategic shift to the individual agency brands and strengthening of their propositions."
EDC UK said it would shift its approach from a group strategy to focus on individual brands within the agency.