The son of a Yorkshire quantity surveyor, Leslie Illingworth was born in Barry, Glamorgan on 2 September 1902, and was educated at Barry County School. He set aside his intention of becoming a hill farmer in order to develop his artistic talents, and began to contribute to the Western Mail while studying at Cardiff School of Art. On graduating from the Royal College of Art, to which he had won a scholarship, he returned to the Western Mail as its political cartoonist but, in 1927, left to work for Punch. On the death of Bernard Partridge in 1945, he became second cartoonist to E H Shepard, and in 1948 joined the Punch table. In 1949 he succeeded Shepard, alternating with Norman Mansbridge until 1968.
Ill at ease with the ‘erudite ideas and inspirations’ of many of the Punch table, he was yet a great friend of Shepard and another Punch cartoonist, Norman Meredith. From 1939, Illingworth was also chief political cartoonist with the Daily Mail., giving way in 1969 to his protege Wally Fawkes (Trog). He later returned to Fleet Street for three years to work as guest cartoonist on the Sun and the News of the World. During the Second World War, he produced work for the Ministry of Defence. His distinctive use of scraperboard was a notable feature of many periodicals during his long and distinguished career. He lived at Robertsbridge, East Sussex. He died on 20 December 1979.