The change follows the resignation of Flic Howard Allen, the previous managing director of retail and leisure, in August.
Howard Allen is to take up the post of head of external affairs at Associated British Foods, the owner of Primark, AB Sugar and Twinings, in January.
Withington, who has been leading the agency’s TUI account for the past year, said: "This is a natural extension for me. I am looking forward to shaping the retail and leisure team over the coming months and years."
Richard Millar, president and chief executive of H+K Strategies UK + Europe, said: "Ian is a brilliant leader of clients and teams. His creativity and track record in developing campaigns that win hearts and minds will be valuable to our stable of clients in the retail and leisure sector. He’s one of H+K’s most accomplished story tellers.
"I would also like to thank Flic for her commitment and contribution to our business and wish her well in her prestigious new role at ABF."
Withington’s expanded role is effective immediately.