Rivoire joins from MW Brands – Europe's leading provider of seafood products – where she was group HR director. Previously, she spent 12 years in senior HR roles at Unilever, working in France and the UK.
She will be based in Millward Brown's London office and will report to the firm's CEO Travyn Rhall.
Rivoire said she was attracted to the role by the large-scale change taking place at the company.
“People strategies can’t simply adapt to this change; they must lead it by having a point of view and by creating a meaningful desire to win together," she said. "I am excited to take on this new role at such an interesting time and I look forward to working with the team to advance our global agenda.”
Commenting on the appointment, Rhall said: “Karen’s unique combination of HR, communications and brand experience, and her success in designing and activating fully aligned talent strategies make her the perfect person for this role."