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 author 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 in 1963.
In 1968 his book Art Deco of the 20s and 30s was published by Studio Vista. This was the first major work on a hitherto neglected period of art, which had been previously been referred to by various names. His use of the term Art Deco became definitive. A year later, also by Studio Vista, he published a book concentrating on the art of caricature from the 13th century to the then present day. The book was simply entitled Cartoons and Caricatures. In 1971 he curated a major Art Deco show at the Minneapolis Institute of Arts.
His major work, the authorised biography of Sir John Betjeman, was published by John Murray in three parts, appearing in 1988, 2002 and 2004. The work had taken him 25 years to research and write. A one-volume abridgement was published in 2006 for Betjeman's centenary. Hillier has also written books on ceramics, posters, cartoons and caricatures. He is a lead reviewer for The Spectator.
In 1983, his work The Style of the Century was published, and it has since been reprinted. The book is a review of the various styles of the 20th Century, from Art Nouveau through psychedelia, pop art and punk.
In August 2006 a rival biography of Betjeman was published by A. N. Wilson, but was discovered to contain a hoax letter, purportedly by Betjeman, but actually containing an insulting acrostic to Wilson. The letter was sent to Wilson by "Eve de Harben", an anagram of "Ever been had?", and the first letters of each sentence, beginning with the second, spelt out the message "AN Wilson is a shit". Hillier was an immediate suspect: 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.