He joins the firm on 4 May, having left Pelham Bell Pottinger in early March following the merger of the two financial agencies.
Paul Downes and David Simonson, founding partners of Merlin, said in a statement: ‘[Cook's] considerable experience in the City and of advising major corporations and institutions across a wide range of sectors at the highest level is an ideal fit with our team.'
Cook became part of Chime Communications after its 2005 acquisition of Grandfield, the City PR shop he had led the buyout of from Saatchi & Saatchi.
He also spent ten years in the international oil industry, principally with BP, where he was head of corporate media relations.
Cook said: ‘Merlin has an excellent reputation in the advisory and corporate community as a highly professional, strategic consultancy with great connections and experience media-wise. We've long been firm but friendly rivals and I now look forward to working with the Merlin team in developing the many opportunities that are available to us.'