Casas was born in Barcelona. His father had made a fortune in Matanzas, Cuba; his mother was from a well-off Catalan family. In 1877 he abandoned the regular course of schooling to study art in the studio of Joan Vicens. In 1881, still in his teens, he was a co-founder of the magazine "L'Avenç"; the 9 October 1881 issue included his sketch of the cloister of Sant Benet in Bages. That same month, accompanied by his cousin Miquel Carbó i Carbó, a medical student, he began his first stay in Paris, where he studied that winter at the Carolus Duran Academy and later at the Gervex Academy, and functioned as a Paris correspondent for "L'Avenç". The next year he had a piece exhibited in Barcelona at the Sala Parés, and in 1883 in Paris the Salon des Champs Elysées exhibited his portrait of himself dressed as a flamenco dancer; the piece won him an invitation as a member of the salon of the Societé d'artistes françaises.