His resignation comes one month after his fellow founding partner Steve Barton left the agency without a job to go to (Campaign, 17 May).
Sources suggest that the agency's future is now in question, with Phil Keevill, the only remaining founder, also looking for a new job.
Keevill said: "For the time being, it will be business as usual as far as possible on what is now a micro business. We'll be using freelancers in the meantime, but clearly we all have to be looking at our various options. It would be daft not to, as you have to safeguard your future."
The departures follow the failed takeover of KBK by the direct marketing and digital agency 20:20 London, which fell through in April after lengthy negotiations.
In his new role, Kershaw will head TDA's creative team across a roster of clients, including AA Insurance, Nationwide and Matalan.
Kershaw said: "It would take a once-in-a-career opportunity to take me away from my agency. TDA is that opportunity. This is a unique chance to work with an excellent team in a direct-to-digital agency that is hungry for growth."
Kershaw, a copywriter, founded Keevill Lee Kershaw in 2003. Following the departure of the founding partner Lisa Lee and the arrival of Barton, the agency rebranded as KBK. Before that, he spent 11 years at Craik Jones Watson Mitchell Voelkel.