Brook was born and raised in Rochester, Kent as Kelly Ann Parsons, the daughter of Sandra Kelly, a cook, and Kenneth Parsons, a scaffolder. She has a younger brother, Damian, and an older half-sister, Sasha. Kenneth Parsons died, aged 57, in Rochester from lung cancer, on 26 November 2007, during Brook's time on Strictly Come Dancing. Brook attended the Thomas Aveling School in Warren Wood, Rochester.
She then studied at the Italia Conti Academy of Theatre Arts in London for three years before becoming a professional model.
Brook's modelling career began at 16 after she won a beauty competition, into which she had been entered by her mother. She later worked on a range of advertising campaigns, including Foster's Lager, Renault Mégane, Walker's crisps, Piz Buin and Bravissimo, a company that specialises in bras and lingerie for large-breasted women. Her figure eventually caught the eye of the editorial team of the Daily Star tabloid, which began featuring her as a Page Three girl.
Brook's picture soon began appearing in other lads' mags such as GQ, Loaded and FHM. In April of the same year (which year??), a poll over 5,000 women for Grazia magazine considered her to have the best British female body. She also topped the 'FHM 100 Sexiest Women in the World' list in 2005, which was said to have polled 15 million people. Appearing in this list every year since 1998, she ranked No.34 in 2008, No.67 in 2009 and No.7 in 2010. She was the cover star of FHM's World Cup 2010 special issue, and was on the cover of the magazine in April 2011.
In February 2007, it was announced that Brook had signed a contract, reported to be worth around £1m, to represent Unilever's Lynx body spray, known as "Axe" in the US and in continental Europe. She has appeared on billboards, in newspapers and on-line as part of their advertising campaign. She has also appeared in commercials for Sky + and T Mobile and modelled for Reebok.
In 2010, she was chosen as the "new face and body" of lingerie maker Ultimo's advertising campaign.
In September 2010, Brook appeared in the American edition of Playboy magazine. In October 2010, Brook appeared live at Clapham Picture House to surprise cinemagoers as part of a promotion for Carlsberg and Sky 3D. In November 2010, Brook presented an award at MTV's EMA's in Madrid.
Brook produces a line of swimwear with New Look, for which she herself has modelled.
In September 2014, Brook launched a clothing line for Simply Be. The following year, she became a brand ambassador for Skechers.
In 1997, aged eighteen, Brook started presenting youth television programmes on MTV, Granada Television and the Trouble TV channel. Brook had a breakthrough into mainstream presenting in January 1999, when she was chosen to replace Denise van Outen as the female half of The Big Breakfast hosting team, alongside Johnny Vaughan. She left the show in July 1999. In 2005, she hosted the reality television programme Celebrity Love Island for ITV.
In 2007, Brook participated in the celebrity dancing competition Strictly Come Dancing on BBC1, with dance partner was Brendan Cole. During the TV series, her father Kenneth Parsons died from cancer, and although she initially decided to continue dancing in his memory, she withdrew from the competition in week nine.
She also competed in the Strictly Come Dancing Christmas Special 2008, dancing the Jive with Brian Fortuna, Brendan Cole having to compete with his later partner Lisa Snowdon. Brook and Fortuna were put into fourth place, but with the studio audience's vote they came second to Jill Halfpenny and Darren Bennett. In 2008, Brook took Jennifer Ellison's place as one of the three judges on the second series of the reality TV programme Dirty Dancing: The Time of Your Life, broadcast between September and November 2008.
In January 2009, she joined the third series of Britain's Got Talent as a fourth judge, but was subsequently axed from the programme after less than a week on the panel, the producers having decided the four-judge format was "too complicated". Brook was billed as a guest judge in the episode in which she appeared, taped in Manchester and aired on 16 May.
In 2013, Brook became a temporary team captain on Celebrity Juice aired on ITV2. She appeared in Season 9, replacing Fearne Cotton while she was on maternity leave. In 2016, Brook served as a team captain on the Channel 5 panel show It's Not Me, It's You.
In 1997, she appeared in a Pulp video "Help the Aged" with Huck Whitney of the band the Flaming Stars, in a slow dance sequence. Brook made her full screen debut with a minor role in the film Sorted. Shortly thereafter, she appeared in the film Ripper. She played the girlfriend of Clark Kent/Superman's nemesis Lex Luthor in four episodes of the American science fiction drama Smallville during the show's first season (2001–02). She has also completed assignments as a film actress in Canada and made a short appearance as Lyle's girlfriend in the 2003 film The Italian Job.
Her first starring role was in School for Seduction, a 2004 film – she received positive reviews for her role, "Seduction offers a few twists on the rapidly degenerating format, not the least of which is the comely Brook in the lead role." In 2004, she played character Nikki Morris in the video game Need for Speed Underground 2, alongside Brooke Burke. In 2005, she appeared in the Philippe Vidal thriller House of 9. In 2005 she starred opposite Billy Zane in the film Survival Island.
Also in 2006, she starred in an Agatha Christie's Marple drama on ITV and appeared as herself throughout the second series of Moving Wallpaper, also for ITV, in 2009. She acted in the science fiction thriller Shadow Play, directed by Nick Simon.
Brook had a leading role in the horror comedy remake Piranha 3D. The film received a worldwide theatrical release on 20 August 2010. Piranha 3D opened to generally positive reviews and commercial success, grossing $83,188,163 on a budget of $24,000,000. Kelly also played as herself in the British comedy film Keith Lemon: The Film.
In 2013 Brook appeared in the video to Lawson's song "Juliet".
In 2014 she was cast as Prudence in an NBC sitcom pilot called One Big Happy, which was subsequently picked up on 9 May.
In December 2000, she played exotic dancer Anya in the play "Eye Contact" at the Riverside Studios in Hammersmith. In October 2008, she returned to the West End as Jeannie, in Neil LaBute's Fat Pig at London's Comedy Theatre. Her performances have had mixed reviews, the Daily Mail said "Kelly Brook has a pencil waist and rabbit-thigh legs, but she is not a great interpreter of a role". In November 2009, she began playing Celia in Calendar Girls at the Noël Coward Theatre, a role previously played by Jerry Hall.
Kelly made a cameo appearance in the cabaret show "Forever Crazy" by Crazy Horse London end of 2012.
In September 2014, Kelly released her autobiography. which reached the Sunday Times Bestseller list on 21 September. Domestic-violence groups and others criticised her during her promotional tour for laughing about punching two ex-boyfriends in the face.
Brook is an animal lover and posed nude with her body painted in scales for the "Whose Skin Are You In?" campaign by PETA.
At end of 2013 Kelly opened her bar Steam and Rye in London. In June 2014 Steam and Rye won Bar of the Year at the London Club and Bar Awards.
Brook met American actor Billy Zane while filming thriller Survival Island in Eleuthera, Bahamas in 2004. Brook and Zane were engaged to be married in mid-2008 and acquired a house in Kent, but Brook postponed the wedding upon the death of her father in November 2007. The couple split in April 2008 and briefly got back together before ending their relationship in August 2008. Brook had been dating rugby player Thom Evans since December 2010. On 16 March 2011, she announced via her Twitter account that she and Evans were expecting a baby girl. On 9 May 2011, Brook suffered a miscarriage. On 1 February 2013, Brook announced through Twitter that she and Evans had split. She began dating fitness model David McIntosh in February 2014 and in March 2014 they announced their engagement; however, on 26 September 2014 she announced via Twitter that the engagement was over and wished him well.