After leaving Barnsbury Park School, he worked as an office boy in a film company. A self-taught artist he became an animator on advertising films. In 1930 he moved to Elstree where he joined the studios of Alexander Korda. He was one of the principal animators of The Fox Hunt (1931), the first British animated colour cartoon with sound.
Giles joined The Reynolds News in 1937 where he drew a weekly topical cartoon and a comic strip, Young Ernie. During the Second World War he produced animated films for the Ministry of Information and his cartoons were reproduced as posters for the government.
In October 1943 Giles transferred to The Daily Express where he joined Sidney Strube in supplying political cartoons. Inspired by the work of Bruce Bairnsfather he also produced the famous Giles Family drawings during this period. Victor Weisz described his work as "a present-day Hogarth". Mark Bryant has pointed out: "He drew without distortion with his cartoon figures set against elaborately detailed naturalistic backgrounds, often with fascinating sub-plots occurring away from the main focus of the picture."
Lord Beaverbrook called him "a man of genius... who takes the solemnity out of the grand occasion and helps the world to keep sane by laughing at its soaring moments". Another fan was Ronald Searle who claimed "in his superb understanding of human behaviour no one can touch him." Arthur Christiansen, his editor at The Daily Express, did not believe his political cartoons were as good as those of David Low: "I do not think that Giles could possibly compete in Low's field. He is not a political cartoonist. Whenever he tries this line of country, he flops badly."