Walter Percy Day was born in Luton (Bedfordshire) to Eli Day and Lucy Day, née Crawley, the second of three children. From 1908 to 1912, he resided in Tunisia, at Sidi Bou Saïd and Tunis, where he pursued a career as a painter of portraits and Orientalist scenes. The dramatic consequences of the “affaire du Jellaz” uprising obliged the family to return to Britain early in 1912.
In 1919, at Ideal Films Studios in Borehamwood, near Elstree Day mastered the art of illusionist techniques. Special effects such as those produced by Day enabled directors to enlarge their repertoire and to tackle subjects which might otherwise have been too costly to produce. In 1922, he relocated to France to its more vibrant cinema. There he introduced the use of the glass shot into French cinema. Used for the first time in Henry Roussel's Les Opprimés, released in 1923, the process was hailed by a critic as a revolution in cinematography. Among the directors with whom Day collaborated during the twenties were Jean Renoir, Raymond Bernard, Julien Duvivier, and Abel Gance. In addition to creating visual effects for Napoléon (1927), Day also played the role of the British Admiral Hood in the film. From 1928, Day's studio became a team, when sons Arthur George Day (1909–1912) and Thomas Sydney Day (1912–1985) began to work for their father, the former as draughtsman and the latter as cameraman and stills photographer, starting with Léon Poirier's Verdun: Visions of History (1928).
In the late 1920s, he learned a new technique while working at the Elstree studio on Alfred Hitchcock's The Ring, that used mirrors and angling to superimpose a miniature over a scene. The inventor, Eugen Schüfftan, whose office was opposite the studios, taught him the process directly.
When shooting the façade of the department store in Julien Duvivier's film Au Bonheur des Dames (1929) proved to be an insurmountable difficulty, Day utilised the stationary matte, a process similar to that patented by Norman Dawn on 11 June 1918.
A meeting with Alexander Korda opened up new perspectives for the Day studio. Day worked with Korda on The Private Life of Henry VIII (1933) a film starring Charles Laughton. Day accordingly established a studio in Iver (Buckinghamshire) and from 1936, directed the matte department at Denham Studios. The artist painted mattes and created trick shots for numerous films by Korda and his stable of directors, who included his brother Zoltan Korda, Anthony Asquith, William Cameron Menzies (Things to Come, 1936), Michael Powell, Lothar Mendes and David Lean. In 1946 Day joined the Korda group as Director of Special Effects at Korda's new premises at Shepperton Studios where he remained until his retirement in 1954.