Pani has been with Millward Brown for five years most recently as country manager of Hong Kong and Taiwan. He will in his new role remain based in Hong Kong.
Borthakur joins from another part of WPP Group's market research business. He was with Research International, part of the Kantar group which Millward Brown is also a member of.
At Research International he was the agency's senior client service director in Singapore. In his new role with Millward Brown, Borthakur will be based in Hong Kong.
Adrian Gonzalez, head of Millward Brown's Greater China region, said: "More than ever it is crucial that we provide our clients with the very best consumer-based advice for building their brands and managing their marketing investments.
"Malini's role as chief client officer will be a valuable addition to this agenda in Greater China.
"Key to this change has been Rupam Borthakur coming on board to replace Malini as Managing Director of our Hong Kong and Taiwan offices.
"Rupam and Malini are both brilliant marketers and will further strengthen our client focus."
Malini said: "My new role will enable me greater focus on our clients and I'm looking forward to helping our teams in the region succeed with their client brand priorities.
"Supporting our client's knowledge with evolving opportunities like digital and cross media is increasingly important."