Mills & Boon is a romance imprint of British publisher Harlequin UK Ltd. It was founded in 1908 by Gerald Rusgrove Mills and Charles Boon as a general publisher. The company moved towards escapist fiction for women in the 1930s. In 1971, the publisher was bought by the Canadian company Harlequin Enterprises, its North American distributor based in Toronto, with whom it had a long informal partnership. The two companies offer a number of imprints that between them account for almost three-quarters of the romance paperbacks published in Britain. Its print books are presently out-numbered and out-sold by the company's e-books, which allowed the publisher to double its output.

Modern Mills & Boon novels, over 100 of which are released each month, cover a wide range of possible romantic subgenres, varying in explicitness, setting and style, although retaining a comforting familiarity that meets reader expectations.

Mills & Boon was founded by former employees of the Methuen publishing house, Gerald Rusgrove Mills (3 January 1877 – 23 September 1928) and Charles Boon (9 May 1877 – 2 December 1943) in 1908 as a general fiction publisher, although their first book was a romance. An early signing was the mystery and crime writer Victor Bridges. Mills & Boon also published—in 1911 and 1912—two early works by Hugh Walpole, including Mr Perrin and Mr Traill, which was later adapted into the film of the same name. Between 1912 and 1923, it published numerous adventure titles by Jack London. In its early years the company also published "educational textbooks, socialist tracts and Shakespeare" as well as "travel guides, children’s and craft books".

It was not until the 1930s that the company began to concentrate specifically on romances.

From the very beginning, Mills & Boon published in a form and at a price that was within the reach of a wide readership. In the 1930s the company noted the rapid rise of commercial circulating libraries and the growing appetite for escapism during the Depression years. Due to these types of libraries, Mills & Boon were able to produce over 160 titles every year during that decade. Historian Ross McKibbin has argued that 'it was the rapid growth of the ‘tuppenny libraries’ in the interwar years which transformed Mills and Boon into a firm which exclusively published romantic fiction.' The favourite genre was romance and the company decided to concentrate on hardback romances, a policy which became increasingly successful. These titles were initially sold through weekly, two-penny libraries. There, they became known as 'the brown books', because of their distinctive binding.

Mills & Boon's innovative marketing strategies also contributed to their novels' popularity. Since 1930, this was evident when the company sent a copy of a chapter of one of their books to whoever formally requested one. Furthermore, the company's innovative marketing extended to creating eye-catching covers, with attention-grabbing colours and "romantic images of couples or the beautiful heroine", to display in store windows.

During World War II, Mills & Boon created ads that notified the public that they could go to their bookseller and place a standing order on any of their books. This guaranteed that Mills & Boons would always keep their novels in stock in bookstores, despite the shortages and other challenges to businesses that resulted from the war. This marketing technique also created a sense of urgency, due to an implication that there was a limited supply for whoever didn't place a standing order. These ads significantly contributed to the company's expansion.

With the decline of commercial lending libraries in the late 1950s, the company's most profitable move was to realise that there would remain a strong market for romance novels, but that sales would depend on readers having easy access to reasonably priced books. As a result, Mills & Boon romance became widely available from newsagents across the country. Mills & Boon began to publish in paperback in the 1960s.