She will start at the company early next year and report into Chris Hirst, the European and UK group chief executive.
Forristal has been tasked with leading talent development for Havas Creative Group.
She has worked at BBH for 12 years and is a trained psychotherapist.
Hirst said: "The best creative businesses are built on a team-based, diverse, and open culture that is constantly evolving and trying new things. Our talent strategy is going to be central to our future success and Nicola will be key to that."