Cook was previously deputy chairman of the Bell Pottinger financial brand and held a key strategic role in the division before the merger.
He was named as a director, alongside previous BPC&F MD Stephen Benzikie, after the merger but will now depart at the end of March.
Cook commented: 'I've greatly enjoyed working with Bell Pottinger Group chairman, Kevin Murray, and his team. I think now that Bell Pottinger Corporate & Financial and Pelham have completed their merger it's the right time for me to move on to fresh challenges.'
He joined Bell Pottinger five years ago when he sold his business Grandfield to Bell Pottinger's parent, Chime Communications. Cook will continue to work with Pelham Bell Pottinger on key clients for a further three months as a consultant.
Murray said: 'Charles made a considerable contribution to Bell Pottinger Corporate & Financial's development and he leaves with our best wishes.'