Grey is looking for a new-business director to replace her, but her management responsibilities will be shared between David Patton, the chief executive, and the managing director, Chris Hirst.
Mendelsohn said: "For some time, I've been considering a change of direction, and with the new team safely on board, the timing is perfect."
Carolyn Carter, the president and chief executive of Grey EMEA, said: "Nicola is a terrific agent of positive change. She has championed our new business both domestically and across Europe, and has helped us create work we are exceptionally proud of. We wish her all the very best."
Garry Lace, the former chief executive of Grey, hired Mendelsohn from Bartle Bogle Hegarty in May 2004. She joined the agency along with Dylan Williams, also from BBH, just weeks after Lace made 48 staff at the agency redundant as part of an agency-wide review.