The French portrait painter Marianne Loir was born into a family with an impressive artistic pedigree. Although her father, Nicolas Loir, was not an artist, the Loirs had been active Parisian silversmiths since the 17th century. Moreover, her brother Alexis Loir III was a highly regarded pastellist and sculptor, also specializing in portraiture, who was based in Paris but who lived and worked as well in England, Russia, and southwestern France.

Unfortunately, little is known about Marianne Loir's life. She was trained by a distinguished French academic painter, Jean-François de Troy, and may have spent some time in Rome. This seems especially likely between the mid-1730s and 1740s, when there is no record of any artistic activity by her in Paris, and when her teacher served as director of the Académie de France in Rome.

Scholars theorize that, like her brother, Marianne Loir may also have lived for a time in the south of France. This theory is based on records of portraits she painted during the 1720s for patrons in Pau and the fact that she was elected to the Academy of Marseilles in 1762. The 10 dated portraits that have been securely attributed to her demonstrate that Loir was an extremely skillful painter of her wealthy and generally aristocratic clients. While sacrificing none of the usual 18th-century emphasis on details of elaborate clothing, Loir also manages to convey an intriguing sense of her sitters' personalities.