Brook was born in Rochester, Kent, England. She is the daughter of Sandra, a cook, and Ken, a scaffolder; she has a younger brother, Damian, and an older half-sister named Sasha. Ken Parsons died from cancer on 26 November 2007, during Brook's time on Strictly Come Dancing.
Brook attended The Thomas Aveling School in Warren Wood, Rochester, Kent. 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 winning a beauty competition, in which she was entered by her mother. She later worked on a range of advertising campaigns, including for Foster's Lager, Renault Megane, Walkers 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 lad mags such as GQ, Loaded and FHM. In April of the same 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. She later ranked 5th in FHM's '100 Sexiest Women in the World 2006', 17th in 2007 and 34th in 2008. She was the cover star of FHMs World Cup 2010 special issue.
In 2006, she signed a contract, reported to be worth around £1m, to present 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.
In October 2010, Kelly Brook appeared live in a Clapham picture house to surprise cinemagoers as part of a promotion for Carlsberg and Sky 3D.
In November 2010, Kelly presented an award at MTV's EMA's in Madrid.
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 program 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 that 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 May 16. In October 2009, Brook claimed that Ant and Dec had been the cause of her removal as 4th judge on Britain's Got Talent. She claimed that she was asked to leave the show after she upset the pair.
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 Warner Bros.' Smallville during the show's first season (2001–2002). She has also completed assignments as a movie actress in Canada and made a short appearance as Lyle's girlfriend in the 2003 movie 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 film House of 9, a thriller about nine seemingly unconnected strangers captured and locked in a house together. The people are forced to compete against one another, in order to survive.
She caused a minor controversy in 2006 opposite Di Pace whilst in the film Survival Island (also known as Three), also co-starring Billy Zane, who would later become her fiancé. Brook requested her nude scenes with Di Pace be excised from the final cut, which the producers refused to do.
Also in 2006, she starred in a Marple drama on ITV and appeared as herself throughout the second series of Moving Wallpaper for ITV1, in 2009. She stars in the Science-Fiction thriller Shadow Play, directed by Nick Simon.
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".
Brook met American actor Billy Zane while filming thriller Survival Island in Greece in 2004. Brook and Zane were engaged to be married in the summer of 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.