Barton will be replaced by Rex Fowler, the KBK business director, who will work alongside the founding partners Phil Keevill and Simon Kershaw. The agency will retain its current title.
Barton's exit comes just three weeks after a planned takeover of KBK by the direct marketing and digital shop 20:20 London fell through.
The privately funded takeover would have seen all KBK staff and assets transferred to a new independent holding company, the 20:20 Group. However, following five months of negotiations, the acquisition failed.
Barton said: "We are doing well, but not well enough, and as a start-up, we have to do better. I need to make more money, and anyone who has to pay school fees will understand my plight. There is nothing more intoxicating than success with a start-up, but it's unlikely I'll do another one.
"I'm talking to people about things from consultancy opportunities to working within agencies and also client-side roles. I'm very open-minded."
Before joining Keevill Lee Kershaw in 2004, Barton had spells at Ogilvy, Leonardo and DLKW Dialogue, and also founded Jones Mason Barton Antenen before its merger into Claydon Heeley.