During the 1930s, Sert co-founded the group GATCPAC (Grup d'Artistes i Tècnics Catalans per al Progrés de l'Arquitectura Contemporània, i.e. Group of Catalan Artists and Technicians for the Progress of Contemporary Architecture), which later was the prominent association, with the addition of the western and north groups, of the GATEPAC (Grupo de Artistas y Técnicos Españoles para el Progreso de l'Arquitectura Contemporánea), which was in turn the Spanish branch of the Congrès International d'Architecture Moderne (CIAM).

Josep Lluis Sert counted amongst his close friends the likes of Alexander Calder, Joan Miró, Georges Braque, and Marc Chagall, for whom he designed studios and homes.

[5] He brought art into the Harvard curriculum through his commissioning of the Carpenter Center and his subsequent avid support for it.

His design for the Fondation Maeght in Saint-Paul-de-Vence, France, the Fundació Joan Miró in Barcelona and the Museum School were more than an architect-client relationship, they were partnerships in the discovery of modern art.

Among Sert's students and colleagues in his studio were leading and past master architects from the United States, Spain, France, Bolivia and Brazil, Venezuela, as well as Dolf Schnebli of Switzerland, Fumihiko Maki of Japan, and Christopher Charles Benninger of India.