Humanity's vulnerability would become the principal theme of Millares' work. As his practice evolved, the abstract and violent textures of his canvases came to be haunted by ghostly traces of human presence hiding in the folds and shadows of his paintings. "The artist records things in their raw state . . . he watches over the despair of our time and stitches up the wounds." (M. Millares, cited in J.-A., França, Millares, Barcelona, 1978). This notion is conveyed by a visceral expression of fragility, fear, and faith.

Millares began making collages in 1954, mixing sacking, ceramic, wood, and sand. It was only after he moved to Madrid in 1955 that he became interested in the work of Alberto Burri, whose works on burlap had a strong effect on him. Often associated with the development of Arte Povera as well as with the Art Informel movement alongside Antoni Tàpies and Jean Fautrier, Millares evolved an aesthetic that is more anchored in the horrors of history—the Second World War, Hirsoshima, and, of course, the Spanish Civil War. Millares was a founding member, along with Antonio Saura, Pablo Serrano, and Manuel Rivera, of the group El Paso, and his work expresses the state of a humanity scarred by deep trauma. "To immediate reality I oppose my protest and my anxiety. It manifests in torn cloth, pierced and wounded surfaces, the noise of strained ropes, the stupid wrinkle of beauty . . ." (M. Millares, cited in Manolo Millares Recent Paintings, exhibition catalogue, Pierre Matisse Gallery, New York, 1960).