Taking on the new title of chief creative officer, Williams will oversee the existing creative directors, Jeff Smith and Simon Butler, and have overall responsibility for the agency's creative output.
David Patton, Grey's chief executive, said: "His job is to orchestrate the department and make sure our product is the right communications solution for the client, no matter what channel that may be.
"I passionately believe we need to integrate all of our disciplines into one central space and reinvent the creative process so we can respond to our clients' business problems with creative communications solutions, not just advertising solutions."
Williams, who has run creative departments at above- and below-the-line agencies, as well as being a digital specialist, added: "This is a huge opportunity to deliver something different, and I am eager to be part of the vision that David and Neil (Hoursten, the executive planning director) share.
"I have run departments before, so I'm not some green 23-year-old computer geek off the street."
Williams joined BMB in January last year after a year at Harrison Troughton Wunderman as the agency's UK online creative director.
Before that, he was the interactive creative director at Publicis Dialog. During his career he has also worked at JWT, WCRS and Saatchi & Saatchi.
- Perspective, page 19.