Riley initially painted representational impressionist works influenced by Henri Matisse and Pierre Bonnard. In 1960, after becoming acquainted with the works of Georges Seurat, she adopted pointillism, in which she painted mostly landscapes. However, that same year she began developing her own artistic language, experimenting with the effect of patterns and contrasts on the viewer. Her international breakthrough came with the inclusion of her work in the legendary Op-art exhibition "The Responsive Eye" at the Museum of Modern Art in New York, for which Brian de Palma filmed a documentary of the same title. A painting by Riley was chosen for the cover of the catalogue. At that time, her paintings were executed in black and white. In 1967 she began to introduce colour into her works. In 1968 Riley was awarded the International Painting Prize at the Venice Biennale.