The announcement was made at an all-staff meeting on Wednesday evening. He is leaving to pursue his writing career but will remain a consultant on some accounts.
Kitcatt said: "I’m excited about the future – mine and the agency’s. Mine because I get to be creative in another way. The agency’s because I believe it’s in a good place and greatness is in its genes."
The CRM agency will not replace Kitcatt’s chief creative officer role directly. The creative department will continue to be managed by Ben Golik, who was promoted to executive creative director in February last year.
Vonnie Alexander, the managing director and last remaining co-founder, still leads the business.
Alexander said: "I am obviously sad to see Paul go. We have worked together for such a long time but the time is right for him and for us to move forward. And we will continue to work together in different ways.
"The agency is going from strength to strength and we will be sharing our plans for the future in the coming weeks. I am confident that Ben will take on the role as creative head effortlessly."
Kitcatt, Alexander, Marc Nohr and Jeremy Shaw launched the agency 13 years ago. Nohr and Shaw left the business last year.
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.
Before Kitcatt Nohr, Kitcatt was the executive creative director at 141 London.