At AMV, she led strategy on accounts such as Guinness globally, as well as leading successful pitches for Virgin Holidays and Smart Energy GB.
Grey has also appointed Jonny Ng, joint head of strategy at Sunshine, as strategy partner. He will lead strategy for the Marks & Spencer account, whose account moved from WPP stablemate Rainey Kelly Campbell Roalfe/Y&R in August.
Ng was joint head of strategy at Sunshine for three years and worked on clients accounts including Boden, Roald Dahl and Warehouse. He has also spent three years client-side at Burberry, where, as global head of engagement marketing, he founded its "runway made-to-order" service where personalised products were available from the catwalk.
Strategy director Ruth Chadwick, who joined Grey in 2013, has also been named deputy head of strategy.
Tanter said: "There are some incredibly talented strategists out there but Lilian and Jonny stand apart in manner, modernity, energy and ambition.
"They are joining an already formidable and diverse strategy crew the heart and soul of which is Chadders. This is a team that are hell-bent on making work that matters in the world. Of any shape. And doing it together."