Mosnier was a student at the Académie de St Luc, Paris, where he trained as a miniature painter. In 1776 he was appointed Peintre de la Reine to Marie-Antoinette. He was approved by the Académie Royale in 1786 and received as a full member in 1788, presenting two portraits of Academicians, the sculptor Charles-Antoine Bridan and the painter Louis Lagrenée I. He exhibited a Self-portrait (St. Petersburg, Hermitage) at the Salon of 1786, showing himself relaxed and confident at the centre of his studio.
After the outbreak of the French Revolution, Mosnier fled to London in 1790 and exhibited at the Royal Academy from 1791 to 1796. His English portraits make some concession to current English taste. From London Mosnier went to Hamburg, where he stayed four years, and then, in 1801, to St Petersburg, a favorite destination for French émigré artists. A potential rival, Elisabeth Vigée-Lebrun, left the city later that year, and Mosnier assumed an influential position. In 1802 he was accepted into the St Petersburg Academy, and he was made a professor there in 1806. His portrait sitters included the imperial family. Mosnier was a versatile and prolific portrait painter, capable of modifying his style in accordance with changed geographical circumstances, and using his skill as a trained miniaturist to good effect in his highly polished and detailed full-size portraits.