Williams is expected to report to a new Omnicom Media Group UK chief executive, who has yet to be recruited.
Williams, 41, will take responsibility for OMD's UK operations, which include the OMD UK agency, Manning Gottlieb OMD, M2M, and OMD International, from 1 January. He has been the managing director of OMD UK for five years, having been at the agency in its various incarnations for 15 years.
Manning, 49, decided to step down after two-and-a-half years following talks with Colin Gottlieb, the chief executive of Omnicom Group EMEA. He will stay on with OMD until next April.
Manning, who launched Manning Gottlieb Media with Gottlieb in 1990 and has spent 27 years working in media agencies, said: "I've decided to declare before getting caught out."
Jonathan Allan, Williams' deputy, is the internal favourite to replace him.
Comment, page 44.