Among the artists he met in this period of his life, and who influenced him, were Laureà Barrau, Santiago Rusiñol, Eugène Carrière, Pierre Puvis de Chavannes, and Ignacio Zuloaga.

Returning together to Paris, they lived together at the Moulin de la Galette in Montmartre, along with painter and art critic Miquel Utrillo and the sketch artist Ramon Canudas.

Rusiñol chronicled these times in as series of articles "Desde el Molino" ("From the Mill") for La Vanguardia; again Casas illustrated.

Casas became an associate of the Societé d'artistes françaises, allowing him to exhibit two works annually at their salon without having to pass through jury competition.

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 emerging modernista art world gained a center with the opening of Els Quatre Gats, a bar modeled on Le Chat Noir in Paris.

The bar hosted tertulias and revolving art exhibits, including one of the first one-man shows by Pablo Picasso; the most prominent piece in its permanent collection was a lighthearted Casas self-portrait, depicting him smoking a pipe while pedaling a tandem bicycle with Romeu as his stoker.

The original of the painting—or most of it: nearly a third of the canvas was cut away by an intervening owner—is now in Barcelona's Museu Nacional d'Art de Catalunya (MNAC); a creditable reproduction resides in the revived Els Quatre Gats.

Like Le Chat Noir, Els Quatre Gats attempted its own literary and artistic magazine, to which Casas was a major contributor.

While his painting career continued successfully through this period, as part owner of a bar Casas engaged heavily in graphic design, adopting the Art Nouveau style that would come to define modernisme.

In 1902, twelve of his canvasses were installed permanently in the rotunda of the Cercle de Liceu, the exclusive private club associated with Barcelona's famous opera house.

Over the remaining years before World War I he traveled extensively in Spain and Europe, sometimes alone and sometimes with Deering, visiting Vienna, Budapest, Munich, Paris, the Netherlands, Madrid, and Galicia.

Up until this time, Casas had kept his distance from the battles of World War I, but in 1918 he visited the front; he painted a self-portrait wearing a military cape.

He continued to paint landscapes and portraits, as well as an anti-tuberculosis poster and others, 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.

Casas contracted tuberculosis, a deadly, wide-spread disease of the time, and after a protracted, months-long illness he died on 29 February 1932 at his home on Carrer Descartes in the Sant Gervasi neighborhood of Barcelona, aged 66.