The daughter of pop singer and poet Guy Béart, Béart was born on August 14, 1965 in the small southern town of Gassin, near St. Tropez. Following her parents' divorce when she was very young, Béart and her siblings were raised by her mother in a small mountain village in Provence. Béart began acting at a young age and had her first substantial role as one of a group of children struggling to survive after a nuclear holocaust in Demain les Momes (1976). A subsequent stint as an au pair in Montreal led to a chance meeting with director Robert Altman, who wanted the unknown actress to appear in one of his upcoming films and encouraged her to continue acting.

The planned collaboration never came to fruition, and, after returning to France, where she began taking drama classes, Béart won her breakthrough role as the vengeful daughter of the late Jean de Florette in Manon des Sources (1986). Following the film's success and her César win, she sought to avoid typecasting, taking on a number of diverse roles in films of varying quality. In 1989, she played a drug addict in Les Enfants du Desordre, while two years later she gained some of her strongest notices as an artist's model in Jacques Rivette's La belle noiseuse.

The following year, Béart starred in what many felt was her strongest film since Manon, Un Coeur en Hiver. She portrayed a high-strung violinist, starring alongside Daniel Auteuil, with whom she starred in Manon and with whom she had been involved with since the mid-1980s; they had a daughter together in 1992 and separated after ten years together.

It was her last highly acclaimed film until 1995, when she starred with Michel Serrault in Nelly & Monsieur Arnaud, playing a dissatisfied young woman who gets caught up in psychological turmoil when she begins working for an emotionally repressed businessman. That same year, she starred with Auteuil in Regis Wargnier's Une Femme Française, in a role written for her by Wargnier. The film, which cast Béart as a passionate woman caught up in a series of love affairs, was not the success it was expected to be, although Béart did win a Best Actress award at the Moscow International Film Festival. Following a window-dressing role in Mission Impossible (1996), her second English language feature, Béart again dedicated herself to making French films. In 1999, she starred in Le Temps retrouvé, Raul Ruiz's acclaimed period drama inspired by the works of Marcel Proust. The film was screened in competition at the 52nd Cannes Film Festival.

In addition to her screen work, Béart is also known in France for her political and social involvement. Aside from being the ambassador for UNICEF, she has made news for her opposition to anti-immigration legislation, making headlines in August 1996 when she was forcibly removed from a siege in a Paris church.