Mastroianni was born in Paris, the daughter of Catherine Deneuve and Marcello Mastroianni. Her elder half-brother is Christian Vadim; her elder half-sister is Barbara Mastroianni.
She had relationships with the actors Benicio del Toro and Andrea Di Stefano. The father of Mastroianni's son Milo (born 31 December 1996) is sculptor Pierre Torreton.
On 11 May 2002, she married the author and musician Benjamin Biolay. Their daughter Anna was born 22 April 2003. Mastroianni and Biolay divorced in 2005.
