Léon Cogniet came to painting just as the pictures by Théodore Géricault (1791-1824) challenged the established neo-classical style and academic hierarchies. Straddling these two worlds, Cogniet experimented with different subjects and composition but generally retained a relatively traditional technique. He entered the École des Beaux-Arts in 1812 and studied with Baron Pierre-Narcisse Guérin (1774-1833), but he was friendly with Delacroix (q.v.), Géricault, and other romantics. Aiming for the most esteemed rank of history painter, Cogniet won the Grand Prix de Rome in 1817 (Helen Freed by Castor and Pollux, École Nationale Supérieure des Beaux-Arts, Paris) and made his Salon debut while studying in Rome (Metabus, King of the Volscians, Musée des Beaux-Arts, Chartres). The French state first bought his paintings at the 1824 Salon (Marius on the Ruins of Carthage, Musée des Augustins, Toulouse), and three years later he was commissioned for a series of the life of St. Stephen for the church of St. Nicolas-des-Champs in Paris (in situ). Around the late 1820s Cogniet became especially enamored of subjects from contemporary British literature, by Sir Walter Scott for example, an interest shared by many French romantic artists. He seems to have been open to different styles and fluctuated between a conventional neoclassicism and more personally inflected romanticism. After 1831 Cogniet became regularly engaged on prestigious public commissions under the regime of Louis-Philippe. He painted Bonaparte's Expedition to Egypt (1833-35), a ceiling in the Louvre palace for the newly created Musée d'Egypte, then several large canvases for the new Musée Historique at Versailles. For these official paintings Cogniet combined his early neoclassical style of clearly organized pictorial space and forms, crisp modeling in light and shade, and smooth surface finish with greater attention to natural-istic details, a looser or more casual composition. The artists adopting this style were called the juste milieu (loosely, "middle of the road"), a term first used in the political arena to identify politicians who espoused neither conservative nor radical ideas but those that, rather, benefited the bourgeoisie, the same class that most admired these paintings. In 1843 Cogniet enjoyed a huge success with Tintoretto Painting His Dead Daughter (Musée des Beaux-Arts, Bordeaux), an invented scene of the great Venetian painter at the deathbed of his beloved daughter. In spite of its popularity, Cogniet exhibited only portraits at the following Salons, and his production fell off after the mid-1850s. He received the highest titles given to painters: Academician (1840), Legion of Honor (1846), and professor at the École des Beaux-Arts (1851).
Teaching became an important aspect of Cogniet's later life. Besides the atelier for men that he established by 1830, he was also responsible for another for women, which his sister Marie-Amélie directed. He served as drawing instructor at the Lycée Louis-le-Grand from 1831 to 1876, where Degas (q.v.) was his student. He also taught at the École Polytechnique for sixteen years, which may have contributed to his interest in such new technologies as drawing machines and photography that he later integrated into his painting. Nevertheless, after the administrative and curricular reforms of 1863 were passed to reorganize the Académie and École des Beaux-Arts, Cogniet resigned from his teaching position there in protest. Among his hundreds of pupils were Bonnat (q.v.), Meissonier (q.v.), and Jean-Paul Laurens (1838-1921).