Leigh, who previously worked at NHS supplier Patientline, departed FTI Consulting after eight years. He joined as group controller and in 2007 became finance director of its UK business, before being promoted to group finance director four years later.
Leigh worked on the sale of Financial Dynamics to FTI in 2006 and on the acquisition and integration of 15 companies around the world.
James Henderson, Bell Pottinger chief executive, said: "The recruitment of Tom as our new CFO underlines the progressive steps we have taken since our MBO two years ago. The experience he will bring to the growth of the business will help us achieve our ambitions to become one of the world’s top communications groups."
Leigh said: "I am delighted to be joining Bell Pottinger at such an exciting time in the development of the business. Its expansion over the past two years has been incredible, with three new offices in Asia in the past year alone, and I look forward to helping the group achieve its global objectives in the years to come."