Eric Salama, Kantar's chief executive, is stepping down in January 2020 after 17 years at the helm of the market research business.
Kantar, which had been wholly owned by WPP until the ad giant sold a majority stake to Bain Capital earlier this year for $4bn, said it was seeking a replacement and that Salama would remain in place in the interim before joining the board as a non-executive director.
Salama, who was stabbed during an attempted robbery of his car in Kew in January, said after a "professionally rewarding" year, it was time to "reflect on my personal priorities and explore other experiences".
"It has been an intense and rewarding 18 months preparing Kantar for sale and successfully closing a deal with our new partner Bain Capital," he said.
"I am confident that Kantar has never been better-positioned for growth than it is now. As professionally rewarding as 2019 has been, a lot has happened on the personal front. After 17 years as CEO, now is the right time for me to pause, reflect on my personal priorities and explore other experiences.
"I am delighted that, after helping with the transition to a new CEO, I will continue to be involved in the business I love."
Mark Read, chief executive of WPP, said: "Eric’s contribution to WPP and Kantar has been immense. Over the past 17 years, he has built Kantar from its infancy into the world’s leading data, insights and consulting company and then worked tirelessly to transition the company to the next phase of its development.
"We understand his decision and are delighted that he will continue to help and advise the company in his new role."
Luca Bassi, a managing director at Bain Capital, added: "I want to thank Eric for his leadership of the business throughout the sale process and his commitment to the company. He has been the true architect of Kantar’s success over the years and has put in place a strategy that will continue that success.
"We fully respect his decision to step down at this stage in the development of the company. We look forward to continuing to work with Eric for the long-term success of Kantar."