Sobieski was born Liliane Rudabet Gloria Elsveta Sobieski in New York City. Her parents are American novelist/screenwriter Elizabeth (née Salomon), who also works as Sobieski's manager, and French painter and former actor Jean Sobieski. Sobieski has a younger brother, Robert, who attends Princeton University.

Sobieski's first name, "Liliane", was the name of her paternal grandmother. One of her middle names, "Elsveta", is derived from "Elżbieta", which is the Polish equivalent of "Elizabeth". Her father is of Polish and Swiss ancestry. Her maternal grandfather, United States Navy captain Robert Salomon, was Jewish, and Sobieski grew up in a "pan-religious" family; she has said that she is "proud of [her] Jewish roots".

She attended Trevor Day School and studied literature and fine art at Brown University.

Sobieski rose to fame in her mid-teens with her appearance in the movie Deep Impact (1998) and played a modern Lolita in Stanley Kubrick's Eyes Wide Shut. Recalling acting alongside Tom Cruise, she said he was "very kind and considerate with me," and says her most vivid recollection of Stanley Kubrick was that he "genuinely seemed to hold something magic".

The title role in the TV movie Joan of Arc (1999) earned her an Emmy nomination and a Golden Globe nomination and a second nomination followed her portrayal of Tosia Altman in the 2001 TV movie Uprising.

Her roles in the Merchant Ivory film A Soldier's Daughter Never Cries (1998), L'Idole (2002) and the miniseries Les Liaisons dangereuses (2003) with Catherine Deneuve and Rupert Everett, an adaptation of Laclos's classic novel of sexual intrigue made use of her fluency in French.

Sobieski speaks fluent French, which she learned from her father, who currently lives in France.

Sobieski is engaged to fashion designer Adam Kimmel. On December 15, 2009, Sobieski gave birth to a baby girl, Louisanna Ray Kimmel.