His departure will break up the longstanding triumvirate with Jonathan Durden, the co-chairman, and the chief executive, David Pattison. PHD was established 12 years ago.
Horswell signed a five-year earn-out contract after the agency bought itself out of Abbott Mead Vickers BBDO and then renewed it for another year. He plans to leave in January 2002.
During his 12 years at the agency, Horswell oversaw internal operations and also played a key part in the development of the PHD group companies, which include Rocket, Compass and Apollo.
Horswell said he was leaving for personal reasons because he has "an itch I want to scratch".
He added: "I've picked up a lot of different lessons over the years, growing both businesses and people. I like the dynamics of small and growing businesses and want to work in a hands-on sense with a number of different companies, rather than in the non-executive chairman role."
Horswell, a managing partner since the launch of the agency, was named as the joint chairman of New PHD in January.
The January restructure also saw Tony Regan, a founder of Michaelides & Bednash, appointed as PHD's first brand strategy director, to sit alongside Morag Blazey, the managing director. Tess Alps, the chairman of Drum PHD, became the deputy chairman.
It has not yet been decided if a joint chairman will be appointed as a direct replacement for Horswell.