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 completion of the merger in January, but will now depart at the end of March.
Cook said: 'I've greatly enjoyed working with Bell Pottinger group chairman Kevin Murray and his team. Now that Bell Pottinger Corporate & Financial and Pelham have completed their merger, it is time for me to move on.'
Murray said: 'Charles made a considerable contribution to Bell Pottinger Corporate & Financial's development and he leaves with our best wishes.'