Williams joined the agency in March last year, along with new-business director Nicola Mendelsohn from Bartle Bogle Hegarty. He took over from Claire Rossi, who resigned not long after now departed chief executive Garry Lace arrived. She subsequently joined WWAV Rapp Collins as a partner.
Williams will leave mid-November and is likely to work on certain projects as a consultant until the end of the year.
A spokeswoman for Grey said that the agency had not begun a hunt for a new planning director, but in the meantime planning duties would be overseen by John Lowery, the former planning director and deputy chairman at Lowe, who joined as chief planning officer for Europe, the Middle East and Africa, in March.
Lowery, who reports directly to Carolyn Carter, the Grey Global Group EMEA president, is based at the London office.
Williams said that he had been thinking of leaving the agency for some time and first spoke to the rest of the management team about his desire to do something else back in July.
According to Williams: "I'm clear on what I want to do next but feel I need a bit of time to work through how best to do it. My career has 20 years to run, I'm playing the long game, this move is part of that. I am immensely proud of the progress the agency has made, and in particular the strength of my department."
Chris Hirst, Grey London's managing director, said: "Dylan has played a crucial role in helping kick-start the changes we set out to make when we arrived. He leaves us having built one of the strongest planning departments in town."
The departure of Williams follows Grey announcing last week that between 7% and 8% of the London agency are to lose their jobs by the end of the year as a result of the network's loss of its above-the-line work on Masterfoods brands.
The cuts follow WPP Group's acquisition of Grey Global, but the job cuts are being attributed to loss of the Masterfoods advertising account rather than Sir Martin Sorrell's takeover.
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