Subject to Financial Conduct Authority approval Platt will join in September after 20 years working for Bupa in a range of senior roles.
David Watson will also become deputy chairman and senior independent director of Countrywide.
Platt said: “I am excited to be joining such an ambitious company as Countrywide and becoming part of the talented team that helps more people than anyone else in the UK move home.
“I am looking forward to working with our award-winning employees to provide a first class service to all our customers and helping the group become even more successful.”
Platt has been managing director of Bupa’s UK, Europe and North America division. More recently she was managing director of Bupa’s international development markets business, growing revenues of over £1bn and leading more than 11,000 people.
She added: “I firmly believe that there are significant opportunities available for Countrywide as an integrated player to further strengthen its business and performance through the recovery of the UK property market.
“I look forward to helping navigate the group through this exciting period of growth for the benefit of our customers, employees and shareholders.”
Grenville Turner, the current group chief executive, added: “I am delighted to be welcoming Alison to the group and to be handing over responsibility to such a proven and energetic individual.
“In assuming the role of non-executive chairman I look forward to continuing to work with the board and supporting the new group chief executive through the next chapter of the group’s journey.”