Forde was previously managing director of P&G United for Starcom Mediavest Group, handling Procter & Gamble, the world’s biggest advertiser, in the UK and Ireland.
She has worked for Publicis Groupe for 18 years, after starting at sister agency Zenith, and was seen as the frontrunner to head Mediavest.
Her promotion is the first senior appointment made by Amanda Morrissey, who became UK chief executive of Publicis Media, the parent company for the group’s media agencies, in April.
Forde said she was looking forward to working with "the fabulous team" at Mediavest, which will continue to be based in the UK from the Turnmill building in London’s Clerkenwell where it shares an office with Starcom.
"It’s an exciting time for Publicis Media and Mediavest as we transform the way we operate for our clients and talent," she said.
Mediavest’s clients include Heineken, Capital One and Royal London Asset Management.
Nigel Waring, who has been leading Mediavest on an interim basis, has been named UK chief operating officer of Mediavest, and Nadine Young will move from sister agency Blue 449 to replace Forde at P&G United.
One of her main tasks will be to shape Mediavest’s identity in the UK market because it has not been a standalone agency brand for a decade and a half.
Forde is already a well-known figure in the London media scene and a frequent judge at industry awards.
Starcom and Mediavest used to be two agencies before they were merged in 2000 – just like sister agencies Zenith and Optimedia, which used to be separate until they were brought together in 2001.
When Publicis Media was founded last December, it disbanded the two media-buying groups, Starcom MediaVest and ZenithOptimedia, in favour of four agencies again.
Forde completes the line-up of Publicis Media’s UK agency chief executives. Pippa Glucklich heads Starcom, Mark Howley leads Zenith and Simon Davis runs Optimedia Blue 449.
Publicis Media said: "Forde will be responsible for leading all client relationships, talent development, driving revenue growth and inspiring her team to produce outstanding work. She will also be responsible for driving the agency’s cultural identity and positioning in the marketplace."
Forde will report to Morrissey and will work with Brian Terkelsen, global brand president of Mediavest UK's parent, Mediavest Spark.
Morrissey said: "Rachel is a proven and successful leader and joins a stellar brand leadership team here in the UK. As a brand-led organisation supported by global practices, our brand CEOs play an incredibly important part in our future success.
"Under our dynamic Publicis Media structure, Rachel and Nigel will drive a distinct agency proposition for Mediavest that will deliver greater business transformation and results for their clients."