In addition, John Wong, previously head of investor relations at Biosensors International, has joined to head the financial practice in Singapore, reporting to Worthington.
Worthington becomes the first MD for Singapore since Shih-Huei Ang, who was latterly promoted to CEO of the Asia division.
The company said the appointments would enable Ang and Asia chairman Piers Pottinger "to concentrate on the further growth of the overall Asian business and explore opportunities in new markets". Bell Pottinger has opened three offices in the region in the past 15 months, with the latest in Yangon, Burma.
Worthington, a former BBC News correspondent, has been a partner in Bell Pottinger’s corporate practice in Singapore since 2012.
Stewart has more than 20 years of experience in the media and public affairs, and spent 10 years at The Times. He has worked in London, Yangon and in the financial sector in Singapore.
Ang said: "Mark has played an instrumental role in the tremendous success of our Asia hub in Singapore and our wider regional business development since he joined two years ago. He has been a key contributor to the excellent work we have delivered for clients and, together with the arrival of Graham Stewart and John Wong, will play a critical role in the further expansion of our business moving forward."
Pottinger said: "Our Asia business is a clear priority for our growth moving forward and we are totally committed to building on our success in the region to date. These new appointments reflect our commitment to both recognise tremendous contributions to our business and add new talent to bring further strength and depth to our team."
Worthington said: "Singapore is without doubt one of the most dynamic and exciting places to do business. In my two years leading Bell Pottinger’s corporate practice, the Singapore office has grown rapidly and developed an enviable portfolio of international and Singapore-headquartered clients.
"I look forward to working with the hugely talented Singapore team to build on our success in corporate and financial communications, and bring clients more of our global, digital and public affairs expertise."