Born Clémence Guichard in Meudon, France, the daughter of Étienne Guichard and a French teacher mother, Poésy took her mother's maiden name 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 CNSAD, the Atelier International de Blanche Salant et Paul Weavor, and 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.

In October 2007, Poésy was announced as one of the spokesmodels for the new fragrance by Chloé. In 2008 she will work on Philip Ridley's Heartless.

She will not reprise her role as Fleur Delacour in the sixth film of the Harry Potter series, as Fleur has not been included in the script. However she may return for the last two films in the series.