Buchanan, who joined Publicis London as chief executive in 2011 from The Red Brick Road, is leaving the agency at the end of the month without another job to go to.
After six years as chief executive, Buchanan stepped up to the chairmanship in 2017, with Poke chief executive Nick Farnhill extending his remit to include Publicis London.
During her tenure, Publicis UK won clients including Morrisons, the International Olympic Committee and Airbus. She is also credited with helping to ensure greater collaboration across Publicis UK agencies including Poke, Arc and August.
Buchanan said: "I am very proud of what Publicis London has achieved in delivering world-class creativity and connected experiences for our clients. Seven years on the agency is in great shape, future fit and I wish them every success, but I’m now keen for a new challenge."
Publicis Worldwide UK will continue to be led by group chief executive Guy Wieynk.
"Over the past seven years, Karen’s leadership has been central to the culture of the business and its transformation to create a stronger, broader and more seamless offering to clients. We will miss her energy and team spirit and wish her all the best in her future endeavours," Wieynk added.