Halling, who is working out a six-month contract at AMV, where she worked on BT. She is expected to start early next year. Thompson resigned from the agency at the beginning of the month.
As a managing partner, she will run one of TBWA/London's four client groups. Hers will include the agency's largest client, Hutchison, and its £45 million account for its yet-to-launch third-generation mobile network 3.
The group will also include the Strongbow, Ask Jeeves and Absolut Vodka accounts.
"The culture of TBWA/London is to grow management from within and it is our intention to groom Katie for executive management," Jonathan Mildenhall, the joint managing director, said.
Thompson spent 18 months at TBWA and is taking six months out to go travelling.
Halling is the first appointment made by Mildenhall and the fellow joint managing director, Matt Shepherd-Smith. They were promoted to managing directors from managing partners as part of the restructure that followed the resignation of the chief executive, Garry Lace, last month.
Andrew McGuinness, who was promoted from managing director to chief executive following Lace's departure, said: "We've been after Katie for some time. It is only now we could create the right position that would allow her maximum contribution towards the future success of the agency."