Although he was born in England, Illingworth’s family moved to Tauranga in 1952, where he found work as a freelance photographer. Returning to England in 1959, Illingworth worked at Victor Musgrave’s Gallery One in London for several years. During this time, Illingworth immersed himself in the vibrant London art scene, resulting in a rejuvenation of both his painting practice and his politics. The painters John Christoforou, Enrcico Baj and Francis Souza had a particular influence on Illingworth, not only in terms of the development of his artistic vocabulary, but also of his anti-authoritarian stance and his contempt for the middle-class values of conservative Kiwi society. Illingworth returned to New Zealand in 1961, and began to produce his characteristic works: colourful, stylised paintings on satirical or social subjects, often featuring doll-like, mouthless figures in simple landscape or interior settings. He exhibited at a number of Auckland galleries throughout the 1960s, including a show at the Barry Lett Galleries which resulted in the police receiving obscenity complaints over his work As Adam and Eve (1965).
In 1966, Illingworth was the recipient of the inaugural Frances Hodgkins Fellowship in Dunedin, where he rekindled his friendship with the poet James K. Baxter. In 1973, Illingworth moved to Coroglen, in the Coromandel Peninsula, where he devoted most of his energy to farming, although he continued to show occasionally throughout the 1970s and into the 80s. Illingworth died of cancer in 1988, at the age of 56. In the years since, he has come to be regarded as an important figure in the history of New Zealand modernism, as well as an icon of the social and cultural upheaval of the 1960s.