Desvallières became acquainted with ancient art during a trip to Italy in 1890, and upon his return began working in the style with which he was most associated, combining dark subjects and violent color with a dramatic conception of religion. He took as his subjects numerous symbolist characters, such as Narcissus (in 1901), Orpheus (1902), and The Marche Towards the Ideal (1903); he also served as one of the founders of the Salon d'Automne. In 1919 he founded the Ateliers d'Art Sacré with Maurice Denis, in an attempt to renew interest in religious art. The atelier served a similar function to that performed by artists' studios in the Middle Ages. Desvallières became interested in religious art after losing a son to World War I in 1915; he himself had commanded a battalion in the Vosges during the war.
Desvallières also tackled a number of public and private decorative programs related to the war; among these were stained glass windows for the Douaumont ossuary and for a church in Pawtucket, Rhode Island. He also illustrated a number of books and plays, including Edmond Rostand's La Princesse Lointaine and Rolla by Alfred de Musset. Until 1950 he also received State commissions.