Born in Paris, Garnier studied under Jean-Léon Gérôme (1824–1904), a demanding teacher renowned for his focus on figure drawing.
Although he never traveled to the East, Garnier visited Spain and the Netherlands, possibly in 1874 with Gérôme, to study the works of Frans Hals.
At the age of 22, Garnier debuted at the 1869 Paris Salon with two works: A Bather and Mlle de Sombreuil Drinking a Glass of Blood. These paintings marked his breakthrough into the art world.
In 1876, he painted The Punishment of the Adulterers, a medieval genre scene with striking use of color. For the 1877 Paris Salon, he created The Favorite, to Whom the Head of the Decapitated Rival Is Brought, inspired by Victor Hugo’s Les Orientales.
In 1879, Garnier produced The Temptation, in which a devout hermit is confronted by two nude female figures testing his conscience.