Millares is associated with the informal movement, which has emphasized gestural experimentation and political engagement, and is seen as largely responsible for the revival of modern Spanish art.
He developed his own visual language inspired by the pre-Hispanic people, whose organic forms painted on the walls of the caves he reconciled with the automatism of the surrealists.
The 1957 Ateneo de Madrid exhibition of his jute canvases and his registration for the Venice Biennale the same year earned him international recognition and the galleries Pierre Matisse and Daniel Cordier signed agreements with him in 1959.
[3] One of the last exhibitions before his death in 1972 and with over 40 paintings and gouaches a comprehensive overview took place from September 24 to November 4, 1971 in the Gallery of Margarete Lauter in Mannheim in collaboration with Juana Mordó.
Posthumous exhibitions have taken place at the Pierre Matisse Gallery, New York (1974), at the Museo Nacional Centro de Arte Reina Sofía, Madrid (1992) and at the Sen-oku Hakuko Kan Museum, Tokyo (2003).