Clémence Poésy was born Clémence Guichard in Paris in 1982. She took her mother's maiden name, Poésy, as her stage name. She attended an alternative school for most of her education, but spent her last year at L'École alsacienne.
She trained at the "Conservatoire National Supérieur d'Art Dramatique", with her first roles being for French TV series between 1997 and 1999, Un homme en colère (1997) and Les monos (1999). Her first feature film was a German production, Olgas Sommer (2002), in 2002 and her second the 2003 French production, Welcome to the Roses (2003).
Her first English speaking feature was as Mary, Queen of Scots, in the 2004 Gunpowder, Treason & Plot (2004) TV movie, for which she won the 2005 FIPA for best actress.
Since then she has starred in many films, the most notable being In Bruges (2008) (2008), which is probably the start of her worldwide recognition, Harry Potter and the Goblet of Fire (2005), Harry Potter and the Deathly Hallows: Part 1 (2010) and Harry Potter and the Deathly Hallows: Part 2 (2011), the US TV series Gossip Girl (2007) and the English TV mini-series, Birdsong (2012) (2012). All of these have shown her to be very capable of roles in multiple languages, periods and roles.
She is known for her natural beauty, devoid of make-up and cosmetics, and she herself says that she does not like using them.