Hillier was born in Redhill, Surrey. His father was Jack Hillier, an authority and author on Japanese art. His mother was Mary Louise Hillier, an authority on wax dolls and automata. Hillier was educated at Reigate Grammar School and Magdalen College, Oxford, where he won the Gladstone Memorial Prize for History. He was employed as a journalist on The Times from 1963. He has since been a reviewer for The Spectator.
In 1968 Hillier's book Art Deco of the 20s and 30s was published by Studio Vista. This was the first major work on a hitherto neglected genre of art that had previously been referred to as Art Moderne (The term Art Moderne has since come to be used to refer to the later streamlined style of Art Deco in the 1930s.). Hillier's use of the term Art Deco became definitive. In 1971 Hillier curated a major Art Deco show at the Minneapolis Institute of Arts, which helped to increase popular awareness of this style.
In 1969 Studio Vista published Hillier's Cartoons and Caricatures, a study of caricature from the 13th century to the late 20th. Hillier has also written books on ceramics and posters, as well as The Style of the Century (1983), a review of the various styles of art in the 20th century, from Art Nouveau through psychedelia and pop art to punk.
Hillier's major work, however, is the authorised biography of Sir John Betjeman. It took Hillier 25 years to research and write, and was published by John Murray in three volumes (1988, 2002 and 2004). A one-volume abridgement was published in 2006 for Betjeman's centenary.
In August 2006 a rival biography of Betjeman was published by A. N. Wilson. It was later discovered to contain a hoax letter, purportedly by Betjeman, but actually containing an acrostic insulting Wilson. The letter had been sent to Wilson by "Eve de Harben", an anagram of "Ever been had?", and the first letters of each sentence, beginning with the second sentence, spelled out the message "A. N. Wilson is a shit." Hillier was an immediate suspect for the literary forgery: the Sunday Times article revealing the hoax was accompanied by a prominent picture of Hillier and noted that an envelope containing a letter supposedly from de Harben to the newspaper had been bought in Winchester, his home town. Hillier initially denied responsibility, but soon admitted that he had written the letter. He explained that he had been angered by Wilson's negative review of the second volume of his biography of Betjeman, and by pre-publication publicity for Wilson's own biography.