Middleton's maternal ancestors, the Harrisons, were working-class labourers and miners from Sunderland and County Durham. Ancestors through her maternal line include Sir Thomas Conyers, 9th Baronet (1731–1810), who was a descendant of King Edward IV through his illegitimate daughter Elizabeth Plantagenet. Other ancestors are Sir Thomas Fairfax (1475–1520) and his wife Anne Gascoigne, who was a descendant of King Edward III.

The Duchess has a number of patronages: The Art Room, National Portrait Gallery, East Anglia's Children's Hospice, Action on Addiction, Place2Be, Natural History Museum, Anna Freud Centre, Sportsaid, and The 1851 Trust. The Duchess is joint Patron of 100 Women in Hedge Fund's Philanthropic Initiatives, along with Prince william and Prince Harry. In 2017, she became patron of The Lawn Tennis Association, All England Lawn Tennis and Croquet Club and Action for Children.

Catherine Elizabeth Middleton was born at Royal Berkshire Hospital in Reading on 9 January 1982 to an upper-middle-class family. She was christened at St Andrew's Bradfield, Berkshire, on 20 June 1982. She is the eldest of three children born to Michael Middleton (b. 1949), and his wife, Carole (née Goldsmith; b. 1955), a former FLIGHT dispatcher and FLIGHT attendant, respectively, who in 1987 founded Party Pieces, a mail order private company that sells party supplies and decorations with an estimated worth of £30 million. The family of her father Michael has ties to British aristocracy and benefited financially from trust funds which they had established over 100 years ago. Her Middleton relatives were reported as having played host to British royalty "as long ago as 1926". She has a younger sister, Pippa, and a younger brother, James.

The family lived in Amman, Jordan, from May 1984 to September 1986; her father worked for British Airways (BA), and Middleton went to an English-language nursery school. Following her return to Berkshire in 1986, she was enrolled aged four at St Andrew's School, a private school near the village of Pangbourne in Berkshire. She boarded part-weekly at St Andrew's in her later years. She then studied briefly at Downe House. She was a boarder at Marlborough College, a co-educational independent boarding school in Wiltshire, and graduated in 2005 from the University of St Andrews in Fife, Scotland, with an undergraduate MA (2:1 Hons) in the history of art. In November 2006, Middleton accepted a position as an accessory buyer with the clothing chain Jigsaw, where she worked part-time until November 2007. She also worked until January 2011 at Party Pieces; her role within the family Business included catalogue design and production, marketing and photography.

In 1997, William's mother, Diana, Princess of Wales, died in a road accident in Paris while being chased by paparazzi. This incident has influenced the Duke's attitude towards media attention. Both the Duchess and her husband have asked that, when off-duty, their privacy should be respected, yet the media has not always gone along with the couple's wishes.

In 2001, Middleton met Prince william while they were both students in residence at St Salvator's Hall at the University of St Andrews. The couple began dating in 2003, although their relationship remained unconfirmed. On 17 October 2005, Middleton complained through her Lawyer about harassment from the media, stating that she had done nothing significant to warrant publicity.

Middleton became prominent for her fashion style and has been placed on numerous "best dressed" lists. She was selected by The Daily Telegraph as the "Most Promising Newcomer" in its 2006 list of style winners and losers. Tatler placed her at number 8 on its yearly listing of the top ten style icons in 2007. She was featured in People magazine's 2007 and 2010 best-dressed lists. Middleton was named as one of Richard Blackwell's ten "Fabulous Fashion Independents" of 2007. In June 2008, Style.com selected Middleton as their monthly beauty icon. In July 2008, Middleton was included in Vanity Fair's international best-dressed list. In February 2011, she was named the Top Fashion Buzzword of the 2011 season by the Global Language Monitor. In January 2012, she was voted 'Headwear Person of the Year.' Middleton was number one on Vanity Fair's annual best dressed lists in 2010, 2011, 2012 and 2013; she also appeared as the cover star in 2012. The Duchess was named to the International Best Dressed Hall of Fame List in 2014. In June 2016, she took part in her first magazine shoot for Vogue's centenary issue appearing on the cover of the magazine. While Catherine wears many new designers, she has also worn dresses by Catherine Walker, who designed many of Diana's favourite evening gowns and day suits.

In April 2007, Prince william and Middleton split up. The couple decided to break up during a holiday in the Swiss resort of Zermatt. Newspapers speculated about the reasons for the split, although these reports relied on anonymous sources. Middleton and her family attended the Concert for Diana at Wembley Stadium, where she and Prince william sat two rows apart. The couple were subsequently seen together in public on a number of occasions and news sources stated that they had "rekindled their relationship".

On 17 May 2008, Middleton attended the wedding of Prince William's cousin Peter Phillips to Autumn Kelly, which the Prince did not attend. On 19 July 2008, she was a guest at the wedding of Lady Rose Windsor and George Gilman. Prince william was away on military operations in the Caribbean, serving aboard HMS Iron Duke. In 2010, Middleton pursued an invasion of privacy claim against two agencies and Photographer Niraj Tanna, who took photographs of her over Christmas 2009. She obtained a public apology, £5,000 in damages, and legal costs.

In 2009, before her engagement to william, Middleton was awarded £10,000 damages and an apology from the photographic press agency Rex Features Ltd after she was photographed playing tennis on Christmas Eve when on holiday in Cornwall.

Prince william and Catherine Middleton became engaged in October 2010, in Kenya, during a 10-day trip to the Lewa Wildlife Conservancy to celebrate Prince William's passing his RAF helicopter search and rescue course. Clarence House announced the engagement on 16 November 2010. Prince william gave Middleton the engagement ring that had belonged to his mother, Diana, Princess of Wales. The couple married in Westminster Abbey on 29 April 2011 (St. Catherine's Day), with the day declared a bank holiday in the United Kingdom. Estimates of the global audience for the wedding ranged around 300 million or more, whilst 26 million watched the event live in Britain alone. On the morning of the wedding day, Prince william was created Duke of Cambridge, Earl of Strathearn and Baron Carrickfergus, with Catherine assuming the feminine forms of the titles.

Another TV film covering similar ground to William & Kate, titled William & Catherine: A Royal Romance and filmed in Bucharest, starred Alice St. Clair and Dan Amboyer as the title characters. Jane Alexander appeared as the Queen and Victor Garber as the Prince of Wales. The film aired on 27 August 2011, in the US on the Hallmark Channel.

In December 2012, two Australian radio hosts, Michael Christian and Mel Greig, called King Edward VII's Hospital Sister Agnes where the Duchess was an in-patient for hyperemesis gravidarum. Pretending to be the Queen and the Prince of Wales, Greig and Christian telephoned the hospital and spoke to a nurse on the Duchess's ward, enquiring about her condition. Following a hospital inquiry and a public backlash against the hoax, the nurse who put the call through to the ward, Jacintha Saldanha, committed suicide. The radio hosts subsequently apologised for their actions.

In September 2013, the Queen granted a conjugal coat of arms to the Duke and Duchess of Cambridge, consisting of their individual arms displayed side by side, beneath a helm and coronet denoting the duke's status as grandson of the sovereign. Below is shown the earlier grant of the duchess's personal arms, impaled with those of her husband.

In 2014, she was already being regarded as a British cultural icon, with young adults from abroad naming her among a group of people that they most associated with UK culture, which included william Shakespeare, Queen Elizabeth II, David Beckham, J. K. Rowling, The Beatles, Charlie Chaplin, Elton John and Adele.

Middleton's father, Michael, and her paternal ancestors were from Leeds, Yorkshire. Her paternal great-grandmother, Olive, was a member of the Lupton family, who are described in the City of Leeds Archives as "landed gentry, a political and Business dynasty"; previously unpublished pictures revealed in March 2015 that Olive Middleton had grown up on her family's Potternewton Hall Estate alongside her cousin, Baroness von Schunck (née Kate Lupton). Middleton's paternal ancestors also include her great-great-grandfather, Politician Francis Martineau Lupton (1848–1921), whose first cousin, Sir Thomas Martineau, was the uncle of World War II Prime Minister Neville Chamberlain.

The Duchess's third pregnancy was announced on 4 September 2017. She was again forced to cancel engagements due to hyperemesis gravidarum. The child is due in April 2018.

The Duchess who attended the 71st British Academy Film Awards did not participate in Time's Up movement which had called for women to wear black on the red carpet. The decision was based on the royal protocols which forbid a member of the royal family to take part in political movements, yet she wore a black sash and carried a black handbag to the event as a variation to the informal black dress code. In March 2018, together with the Countess of Wessex, the Duchess hosted the Commonwealth Fashion Exchange reception at Buckingham Palace during the 2018 London Fashion Week.