Rayment will join in April in a role that spans internal and external comms across Publicis Media. The group has more than 2,000 staff in the UK, at the agencies Digitas, Performics, Spark Foundry, Starcom and Zenith.
She fills a vacancy left by Rupert Smith, who joined Newsworks last September.
Rayment spent more than 12 years at Dentsu Aegis Network, most recently serving as its director of communications and marketing for the UK and Ireland.
She will report to Publicis Media chief executive Sue Frogley.
"Mandy is highly respected in her field and brings years of industry experience that will be invaluable as we look to build on our recent successes," Frogley said. "Our paths have crossed before, so I'm thrilled to be working with her again and welcome her on board."
Rayment added: "I have enjoyed 12 wonderful years at Dentsu Aegis Network, but the opportunity to join Sue and the team at Publicis Media, to help support their ambitious plans, was too good to refuse. It's a really exciting time to be joining and I can't wait to get started."
Rayment's appointment follows a series of account wins for Publicis Media agencies, from clients including Three, Disney and Macmillan Cancer Support.
At Dentsu Aegis Network, Rayment led marcoms and was a founding member and co-chair of its LGBT+ network, playing a role in driving the group's broader diversity and inclusion agenda.
Dentsu Aegis Network is undergoing a global restructure that could affect as many as 400 jobs in the UK, either in terms of redundancies or relocation of staff from London to other parts of the country.