She appeared on the Fox drama "The O.C." as Julie Cooper, a cunning socialite who marries for money. Her character in the show is mainly portrayed as a manipulative, morally wrong woman, but as the show progresses the reasons behind her choices are explained. She was originally billed as a guest star in the first few episodes, and due to fan response was offered a series regular contract; Clarke accepted the role and her character became an integral part of the show's storylines. As well as starring on "The O.C.", she has appeared on HBO's hit series "Entourage" as a fictionalized version of herself, married to Malcolm McDowell's Terrance McQuewick. Clarke also appeared as the Devil's girlfriend in The CW's 2007 series "Reaper". She also auditioned for the role of "Number Six" on SCI-FI's smash hit "Battlestar Galactica" but the role ultimately went to Tricia Helfer.