Susan Day is the granddaughter of Walter Percy Day and niece of Peter Ellenshaw, both matte shot and special effects artists. She was born in Uxbridge, near London, in 1943 to the cameraman and photographer Thomas Sydney Day and Irene Winifred Day, née Hook, raised in Sunbury-on-Thames and educated at Ashford Grammar School (Middlesex). At the age of sixteen, she emigrated with her parents and sister, the artist and author Sonia Day to the West Indies, to Jamaica and later the Bahamas. Various jobs included a stint decorating ceramic plates at the Bahamian branch of the Chelsea Pottery, run by the artist David Rawnsley (1909-1977).
In 1968, Day settled in Paris, where she formed lasting friendships with the artists Richard Davies (1973-1991) and Holley Chirot (1942-1984). She read Islamic art at the Ecole du Louvre, graduating in 1985 with a thesis devoted to the Ottoman Turkish carpet collection of Les Arts Décoratifs. From 1981 to 2004 she worked for the Institut Français d'Architecture/Cité de l'architecture et du patrimoine, as Chief Librarian. During this period she wrote monographies on the architect-decorators Louis Süe and Jean-Charles Moreux, and organized an exhibition on the work of the former, in addition to contributing notices to the Macmillan Dictionary of Art and the Allegemeines Künstlerlexikon.
Concurrently, Day remained active in the field of textiles, serving as Scientific Advisor to an exhibition of oriental carpets held at Les Arts Décoratifs in 1995-1996 and writing various books and articles on the subject. In publishing Art Deco and Modernist Carpets (Thames and Hudson, 2002) she was able to pool her twin fields of knowledge. In 2004 Day joined the Islamic Department of the Louvre Museum, as Scientific Advisor for carpets and textiles.
Since her retirement in September 2007, Day has remained active as an art historian, working as a Consultant for the Musée des Tissus, Lyon and Les Arts Décoratifs, Paris. A member of the academic committee of the International Conference on Oriental Carpets (ICOC) and the Centre International d'Etudes de Textiles Anciens (C.I.E.T.A.). As a painter, Day is friend of the artist Rosy Lamb and attended courses at the Académie de la Grande Chaumière.