Pattison, 50, who launched PHD in 1990 alongside Nick Horswell and Jonathan Durden, has handed his notice in to Omnicom management. He will leave PHD next year.
He said: "I've been at PHD for 17 years, I've done six years since the end of my earnout from Omnicom and PHD could not be in better shape, so it feels the right time to move on."
Pattison's decision follows the roll-out of PHD as a global network this year. Pattison is also the IPA president after taking on the two-year term in April 2005, with an agenda of promoting transparency between agencies and clients.
He said that he is talking to Omnicom about other jobs, and hasn't discounted staying within the holding company but is looking at other options.
Pattison, who alongside Durden was the joint media director of WCRS before launching PHD, helped the agency build a reputation as a trailblazing, planning-led shop and, in 1996, was one of the architects of its £5m sale to Abbott Mead Vickers, which became part of Omnicom two years later.
He then went to the US to launch PHD and spent several years attempting to secure Omnicom's backing for a wider roll-out of the network.
Pattison's departure will leave Durden as the only remaining founding partner with an active role in the business. Horswell left in 2002.
Earlier this week, PHD announced that Morag Blazey had been promoted from managing director to the role of chief executive.
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