Riley was born in the London borough of Norwood, the daughter of a merchant. Her early years were spent in Cornwall and Lincolnshire. From 1949 to 1952, she attended Goldsmiths’ College, and from 1952 to 1955, she attended the Royal College of Art. She began painting figure subjects in a semi-impressionist style before transitioning to pointillism around 1958, primarily making landscapes. She developed a style in 1960 that investigated the dynamic potentialities of visual phenomena. Op-art pieces, such as Fall, 1963, have a disorienting physical effect on the sight.