Poésy was born in Meudon to Étienne Guichard and a French teacher mother, whose maiden name Poésy took as her stage name. Her father gave Poésy her first acting job when she was 14; she had two lines. She has a younger sister, Maëlle, who is also an actress.

Since leaving La Source, the bilingual and alternative school she attended, at the age of 16, she has studied drama at the university of Nanterre and appeared in a number of French film, television, and stage roles. Her first English-speaking role was in the BBC mini-series Gunpowder, Treason & Plot where she played Mary I of Scotland.

She is currently working on the film In Bruges, due for broadcast in 2008, and is slated to appear in the sci-fi film Tender Interface. In addition to this, she has appeared on the April 2007 cover of i-D magazine.