The pair will be key players in a radical restructuring and repositioning of Grey by its chief executive, Garry Lace. Lace's coup, a massive blow to BBH, is part of his aim to grow the agency and strike a more even balance between domestic business and the giant international accounts that provide the lion's share of its income.
Mendelsohn replaces Jo Smith, who resigned last month, while Williams takes over from Clare Rossi, who has taken a senior role at WWAV Rapp Collins.
Lace, Mendelsohn and Williams will form the agency's core team along with Dave Alberts, who joins as the executive creative director this summer from Mojo-Partners in Sydney.
Lace's radical programme of change has already involved the loss of 48 jobs. However, the agency has overcome a barren new-business period by scooping Visa International and the consolidated Dairy Crest accounts.
Mendelsohn, 31, who has spent her entire career at BBH, said she had not been looking for a move until Lace persuaded her to make the switch.
"People say Grey can't be turned around but Garry believes it can and so do I," she said. "New business is already being won and the new management team represents a huge opportunity to re-energise the agency."
Lace said: "Nicola was the only name I had in my mind for the job. Not only is she very well known within the marketing community, but she will help us deliver on our intention to get a more even distribution between global and domestic accounts."
For Williams, 30, a BBH board planner, his decision to join Grey comes after rejecting an earlier offer from Lace to join him at TBWA/London.
"Dylan will provide the agency's intellectual backbone as well as a degree of synergy with Dave," Lace said.
BBH has assigned Hugh Baillie, the group business development director, to oversee for Mendelsohn until a successor is appointed.