She will spend some time with her family, before looking for new opportunities next year.
Richard Madden, the chief strategy officer, is also leaving. Madden has been the agency's chief strategy officer since 2011 and was the planning director for four years before that.
The news comes three months after Paul Kitcatt, another of the agency’s founders, left to pursue a writing career.
Kitcatt Nohr has promoted Hattie Whiting, a client partner at DigitasLBi, to be the managing director of the agency. Ed Beard, currently the head of creative strategy at DigitasLBi, also joins the agency as the chief strategy officer.
A spokesman for DigitasLBi, the CRM agency's parent group, said Kitcatt Nohr will continue to keep its name and be run as a separate entity.
Ben Golik, the executive creative director of Kitcatt Nohr, and Sue MacLure, the group data strategy partner, will be the other members of the agency’s management team.
Kitcatt Nohr’s clients include Bupa, Nissan and Sky.
Michael Islip, the UK chief executive of DigitasLBi, said: "I’d like to thank Vonnie and Richard for their dedication and hard work building Kitcatt Nohr into the market leading agency it is today.
"With someone of Hattie’s calibre at the helm of a first-rate management team, Kitcatt Nohr will go from strength to strength as it continues to forge new ground in customer engagement."
Kitcatt, Alexander, Marc Nohr and Jeremy Shaw launched the agency 13 years ago. Nohr and Shaw both left the business in 2013. Nohr has since become the chief executive of Fold7.
The shop, originally called Kitcatt Nohr Alexander Shaw, became Kitcatt Nohr Digitas in 2011 after its sale to Publicis Groupe. It dropped the "Digitas" name in 2014, two years after Digitas was merged with LBi.
Alexander said: "Kitcatt Nohr is agency with such a strong reputation, a very defined culture and a clear vision for the future as part of the DigitasLBi family.
"The time is right for a new leadership team to take the agency forward and for me to spend some time with my family. I wish them well and I will enjoy watching their success from afar."