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 next few years he continued to paint and travel, spending most autumns and winters in Paris and the rest of the year in Spain, mostly in Barcelona but also in Madrid and Granada. His 1886 painting of the crowd at the Madrid bullfighting ring was to be the first of many highly detailed paintings of crowds. That year he survived tuberculosis, and convalesced for the winter in Barcelona.
With Rusiñol and with sculptor Enric Clarasó he exhibited at Sala Parés in 1890. His work from this period, such as Plen Air and the Bal du Moulin de la Galette lies somewhere between an academic style and that of the French impressionists. The style that would become known as modernisme had not yet fully come together, but the key people were beginning to know one another, and successful Catalan artists were increasingly coming to identify themselves with Barcelona as much as with Paris.
In 1922, Casa finally married Júlia Peraire, and in 1924 she came along with him on a trip to the United States, during which he once again made portraits of the rich and famous.
By the 1920s, Casas had fallen far away from the avant-gardiste tendencies of his youth. If anything, his work from this period looks like it came from an academic painter of an earlier time than his work of the 1890s. He continued painting landscapes and portraits, as well as anti-tuberculosis posters and the like, but by the time of his death in 1932, shortly after the emergence of the Second Spanish Republic, he was already more a figure of the past than the present.