After starting on the stage as a child, Poésy studied drama and has been active in both film and television since 1999, including some English-language productions.

[5] After leaving La Source aged 16, Poésy studied drama at the Conservatoire National Supérieur d'Art Dramatique (CNSAD, the French National Academy of Dramatic Arts), the Atelier International de Blanche Salant et Paul Weaver, and Paris Nanterre University.

[6] Poésy's first English-speaking role was in the BBC mini-series Gunpowder, Treason & Plot (2004), in which she portrayed Mary, Queen of Scots, subsequently winning the 2005 Golden FIPA for actress in a TV Series and Serial.

In 2008, Poésy starred in the Academy Award-nominated film In Bruges, alongside Colin Farrell, and Harry Potter co-stars Ralph Fiennes and Brendan Gleeson.

[9] She played Chuck Bass's new French girlfriend, Eva, in the fourth season of the CW hit show Gossip Girl.

[11] In 2011, she appeared alongside Rupert Friend in Lullaby for Pi, a romantic drama and Benoit Philippon's directorial debut.

[13] She worked alongside Michael Caine in Mr. Morgan's Last Love,[14] and in 2012 made her Broadway debut in Cyrano de Bergerac as Roxane.