The announcement comes 18 months after Grey raided Bartle Bogle Hegarty to capture Williams, then a board planner, and Nicola Mendelsohn, now Grey London's deputy chairman.
Williams said he was leaving because he wanted to apply his planning skills to other forms of communication.
"That could range from catwalk shows to pop promos," he added. "It means I'll either have to join a different kind of company or start one of my own."
His departure is said to be unconnected either with the agency's recent loss of its Masterfoods business or Grey Global Group's acquisition by WPP.
"I told the agency management team in July that I wanted to leave and I've effectively been working my notice since then," Williams said. "When the Mars loss was announced, we thought it an appropriate time to make it public."
John Lowery, the chief planning officer for Europe, the Middle East and Africa, will caretake the London agency's planning department until a permanent successor is found.
Williams, who will continue working on certain projects until the end of the year, was hailed as Grey's new "intellectual backbone" when hired by its then chief executive, Garry Lace.
The Grey London managing director, Chris Hirst, said: "Dylan leaves us having built one of the strongest planning departments in town."