Caryl Dexter, who has run the agency since 2002, joins P&MM as a sales director.
The agency will be known as Peppermint at P&MM as an "interim brand" until the end of the year to provide continuity for the market, said P&MM managing director Nigel Cooper.
"I have spoken to Caryl for four or five years about acquiring the business and she finally said yes," said Cooper.
Dexter has previously held senior positions at Brevis and Opus. Former Peppermint staff Natasha Duff and Julie Weyms also join P&MM.
"I am delighted to be joining P&MM and look forward to working with what is regarded as the best team in the business," said Dexter.
"I have known Caryl for nearly ten years and long admired her knowledge, integrity, creativity and client relationships, so I am thrilled that she will be working with us going forward. Peppermint is a great addition to our business, bringing existing clients and creating new opportunities," said Cooper.