Illingworth was one of the best-known and respected political cartoonists of the 20th Century, and one of a handful of Welsh cartoonists to have gained international acclaim.
Having studied art at the City of Cardiff Technical School (later known as Cardiff Art School), the Royal College of Art, London, and the Slade School of Art, Illingworth started his career as a cartoonist at the Cardiff-based Western Mail.
In 1939 he joined the Daily Mail, where some of his most famous cartoons were drawn during the Second World War, commenting on both Chamberlain and Churchill's leadership as well as Allied military victories. After the war he became Chief Cartoonist for the satirical magazine Punch, but remained working for the Daily Mail until his retirement in 1969.
In 1962 he was voted political and social cartoonist of the year by the Cartoonists' Club of Great Britain and three years later given a special award for distinguished services to cartooning in 1965. In 1966 Illingworth was the first President of the British Cartoonists' Association when it was founded.