Fisher, who currently runs the agency's Unilever business, comprising the Knorr, Marmite and PG Tips brands, and British Bakeries' Hovis account, replaces Philippa Roberts, who has been made the business director.
James Best, the chairman of DDB London, said: "Karen has more than demonstrated her ability to manage some of our largest and most complex account teams and she does so with wit and style. I have every confidence she will have great success in bringing her skills to bear in running the largest department in the agency."
Fisher, who was previously the deputy head of account management, will be responsible for all recruitment and training of account staff at the agency. "It is a brilliant opportunity for me to shape the department to answer the requirements of our increasingly diverse clients," Fisher said. "As we grow our international and local client base, the demands on the department are constantly changing."
The board account director Jon Busk, who runs the Volkswagen business, has been promoted to deputy head of account management.