Baumann, who joined Leo Burnett in 2001, is not leaving to take up another role. However, it is understood that it was her decision to leave.
The agency will not be appointing a direct replacement, with the leadership team to be headed by chief executive Gareth Collins, chief strategy officer Josh Bullmore and chief creative officer Chaka Sobhani.
Baumann, who started her career at J Walter Thompson, held a number of senior roles at Leo Burnett, including head of account management and group talent strategy director, and spent three years leading the agency’s luxury division Atelier.
Commenting on the departure, Collins said: "Sarah has been an incredible asset to Leo Burnett over the last 16 years, we thank her for her contribution and wish her all the best with her next endeavour."
Collins joined Leo Burnett London in October as part of a restructure that aligned the agency with Fallon. That move followed the exit of the agency's previous chief executive, Paul Lawson, who joined Y&R London.
Baumann added: "I have had an amazing 16 years at Leo Burnett, helping grow and evolve the business. I wish the agency every success as it evolves into its next chapter."