Grimmett was head of strategy at Starcom, for brands including Honda, Samsung, Kraft Heinz and Lidl, for which she played a key role in winning in 2015.
At Havas she has been tasked with leading strategic output across Havas Media, Arena, and Team O2 (the dedicated shop that handles O2 media, previously known as Forward).
She had previously worked at MPG, during its merger into Havas, and has also worked at Mindshare, UM and Vizeum.
Havas Group Media’s client roster also includes Domino’s, Emirates, Hyundai/Kia, Pernod Ricard and Royal Mail.
Grimmett will report to Matt Adams, chief executive of Havas Group Media UK, and be in charge of strategy leads within the agencies.
Adams said: "Eva raises the ceiling in every way – a fantastic talent who makes tangible difference the people and clients she works with. It’s great to welcome her back to Havas."