Leona Lewis was born in the London Borough of Islington, to parents Aural Josiah "Joe" Lewis, an English youth worker of Afro-Caribbean Guyanese descent, and Maria Lewis, a Welsh social worker of Italian/Irish descent . At the age of five, she attended the Sylvia Young Theatre School, and later the Italia Conti Academy and the BRIT School. There she learned guitar and piano and also worked hard to write her own songs in order to be taken seriously as an accomplished musician.

She wrote her first song at the age of twelve, and won a number of talent competitions. After leaving school, Lewis took a number of jobs to pay to get into a recording studio, including waitressing and several receptionist jobs. Lewis also has said her biggest idol as a singer growing up was the African-American soprano Leontyne Price.

She wrote, recorded and produced a number of songs including two demo albums, Twilight and Best Kept Secret. When she was fifteen years old, she recorded a version of Minnie Riperton's "Lovin' You" with music producer Marley J. Wills, who commented that "Leona could hit the notes and I think she did it better than the original." At one point, Lewis went to USA to try start her career and met with producer and American Idol judge Randy Jackson , who persuaded her to give UK another try and enter a competition. Unable to secure a contract, Lewis considered placing her music career on hold to attend university, before her boyfriend persuaded her to listen to Randy Jackson and the other producers she talked to and to enter The X Factor.

In 2006, Lewis auditioned for the third series of The X Factor, singing "Over the Rainbow". After singing tracks such as 'Hero', 'Without you' and 'Baby one more time' at the bootcamp and judges' houses stages, she made it through to the final stages and was announced the winner on 16 December 2006, winning a £1 million recording contract, therefore becoming the first female winner of The X Factor and the third winner after Shayne Ward and Steve Brookstein. She was mentored by Simon Cowell.

Lewis' voice was consistently praised by all three judges — Simon Cowell, Sharon Osbourne and Louis Walsh. Consistently, she was compared with divas such as Christina Aguilera, Whitney Houston, Mariah Carey and Celine Dion. Lewis was also criticised, by some of the judges and in the press, for her lack of self-confidence. Vocal coach Mark Hudson also criticised a lack of star quality. On the final night of the show, Take That joined Lewis on stage for a duet of their song "A Million Love Songs". After the performance, singer Gary Barlow told Cowell, "you've got a big responsibility because this girl is probably fifty times better than any contestant you have ever had on this show, and it's your responsibility to make her the best record you can, so please do that."

On the Oprah Winfrey Show on the 17 of March 2008, Simon Cowell said that it was during Lewis' performance of "Summertime" in the third live round of The X Factor (broadcast 28 October 2006) that he "could see her transform from a great singer into a superstar".

Over one-million copies of the winner's single "A Moment Like This", were pre-ordered by stores, although at the time, the winner of the show had not been announced and it was unknown who would be releasing the single. "A Moment Like This" was released on CD on 20 December 2006, and was available as a digital download from midnight on 17 December. It broke a world record after it was downloaded fifty-thousand times in thirty minutes. On 24 December, "A Moment Like This" was crowned the 2006 UK Christmas number-one single, having sold 105,381 copies, beating Take That's single "Patience", and outselling the rest of the Top 40's sales combined. It was reported that one chain was "shifting more than 100 of her CDs a minute". The single became the biggest-downloaded song in 2006, beating Gnarls Barkley's "Crazy". The single stayed at number one for four weeks, and also stayed at the top spot in the Irish Singles Chart for six weeks. It went on to sell in the region of 1,000,000 copies.

A four-track download EP, "It's All for You", that she recorded before entering The X Factor was released digitally on 18 December 2006, through The Schizofreniks record label. The single was banned from several online stores by Lewis's record label, Sony BMG, and was therefore unable to chart.

In February 2007, Lewis signed a £5 million ($9.7 million) five album contract in the United States with Clive Davis's record label, J Records, and showcased for several American music executives at the Beverly Hilton Hotel in Beverly Hills, California.

On 16 January 2007, "A Moment Like This" was shortlisted for the British Single award at the 2007 BRIT Awards. It made it to the second round but was eliminated and did not make it to the final selection. In May, it was awarded the Ivor Novello Award for Best Selling British Single.

On 25 April 2007, a press release was sent out revealing that Simon Cowell and Clive Davis would work together in a first-of-its-kind partnership on both the song and producer selection for Lewis' debut album, entitled Spirit. On 24 September 2007, Lewis performed a special album launch party at the Mandarin Oriental Hotel in Knightsbridge, London, where she performed four tracks from the album. The album was released in Ireland on 9 November 2007, where it entered the Irish Albums Chart at number one, being Ireland's fastest-selling debut album of all time. It was released on 12 November 2007 in the United Kingdom, where it entered the UK Albums Chart at number one, and was also the UK's fastest selling debut album, and the fourth fastest selling album of all time.

The album was released in several other nations in January 2008, and went to number one in New Zealand, Australia, Austria, Germany, South Africa and Switzerland. Spirit was released in the United States on 8 April 2008, and entered the Billboard 200 at number one.

Lewis recorded tracks for the album in London, Miami, Los Angeles, New York City, and Atlanta. She worked with several songwriters and record producers including Dr. Luke, Dallas Austin, Max Martin, Steve Mac, Stargate, Walter Afanasieff, Salaam Remi, Kara DioGuardi, Per Magnusson, David Kreuger, Richard Page and Novel. She also recorded "I'm You", which was written and produced by Ne-Yo. Two further tracks were recorded in early 2008 for the U.S. release of the album, "Forgive Me" which was produced by Akon and "Misses Glass" which was produced by Madd Scientist.

Lewis' second single, "Bleeding Love", produced by One Republic's Ryan Tedder, and written by Tedder and Jesse McCartney, was released on 22 October 2007 in the UK. Lewis performed the song on the first live show of the fourth series of The X Factor on 20 October. The single sold 66,000 copies on its first day of release and went on to sell 218,805 copies in its first week, giving it the biggest first-week sales of 2007 to date. It entered the UK Singles Chart at number one, where it stayed for seven weeks. It also debuted at number one in the Irish Singles Chart, remaining there for eight weeks. Elsewhere, it reached number one in the singles charts of New Zealand, Australia, France, Germany, Norway, Switzerland, Belgium, The Netherlands, Austria, Switzerland, Canada and the United States. The video for the single became the third most viewed music video on Youtube, and the fourth most viewed overall. As of 1st January 2009, it has 78.5 million views, topped only by Avril Lavigne's "Girlfriend" (111.6m views) and Chris Brown's "With You" (82.3m). On 16 December 2007, "Bleeding Love" won The Record of the Year 2007 award.

On 31 October 2007, Lewis appeared on BBC Radio 1's Live Lounge, with acoustic performances of "Bleeding Love" and a cover of Snow Patrol's 2003 single "Run". Her version of "Run" was popular with listeners, and as a result, was added to Radio 1's A list (a list of songs receiving the most airplay). On 6 November she won 'Newcomer of the Year' at the Cosmopolitan Ultimate Woman of the Year Awards. In the 30 November 2007 edition of American magazine Entertainment Weekly, Lewis was included among "8 to Watch in '08" in their 2007 "Entertainer of the Year" issue.

On 14 January 2008, Lewis was nominated for four BRIT Awards, in the categories British Female Solo Artist, British Breakthrough Act, British Album for Spirit, and British Single for "Bleeding Love". She also performed at the awards ceremony on 20 February 2008. Despite being the favourite to win the most awards, she received none.

On 9 February 2008, Lewis performed at Clive Davis's annual pre-Grammy party, as Davis's 'Special New Artist' guest. She also made her first US television debut on The Oprah Winfrey Show, on 17 March 2008. In February 2008, Lewis broke the major American charts as "Bleeding Love" entered the Billboard Hot 100 at number 85 and then went on to peak at number one for four non-consecutive weeks beginning on the 5 April 2008. "Bleeding Love" became the first track by a UK female to hit number one since Kim Wilde 'You Keep Me Hanging On' in 1987. Lewis is also the third British female to go to number one with a debut single, the other two acts being Petula Clark with "Downtown" in 1965 and Sheena Easton with "Morning Train (Nine to Five)" in 1981, and only the sixth British female to ever top the US singles chart, following Clark, Easton, Wilde, Lulu and Bonnie Tyler. "Bleeding Love" is only the second single to ever fall out and return to the number one position on the Hot 100 twice, returning to number one on the charts dated 26 April 2008 and again on 10 May 2008. The first was "Le Freak" by Chic in 1978 and 1979.

Lewis' third single, a double A-side featuring "Better in Time" and "Footprints in the Sand", was released in the United Kingdom in March 2008, in aid of Sport Relief, and she visited South Africa for the charity. The songs reached a peak of number 2 in the UK singles chart selling over 40,000 copies in its first week of physical release. She was kept off the number one position by Duffy who sold only 302 more copies of her single "Mercy" than Lewis. Better In Time was released as Lewis' second single in the U.S. on 15 July 2008. It reached a peak of number 11 on the U.S. Billboard Hot 100 chart. As a part of Sport Relief, Lewis has set up a website, 'yougivemestrength.com', for people to share their inspirational stories.

On 27 June 2008, Lewis performed "Bleeding Love" and "Better In Time" at the 46664 Concert in Hyde Park, London to celebrate the 90th Birthday of Nelson Mandela. She also performed at the MTV Asia Awards 2008 in Genting Highlands, Malaysia where she won the award for Breakthrough Artist. On 24 August 2008, Lewis performed "Whole Lotta Love" with guitarist Jimmy Page of Led Zeppelin at the 2008 Summer Olympics closing ceremony in Beijing, representing the handover to London as the host of the 2012 Summer Olympics. Her performance was watched by a worldwide TV audience of around three billion people.

On 5 September 2008, Lewis joined Mariah Carey, Carrie Underwood, Rihanna, Ciara, Beyonce, and many other female singers to perform a single for the anti-cancer campaign Stand Up to Cancer. The single, titled "Just Stand Up", was performed live during the one-hour telethon that which aired on all major U.S. television networks. Two days later, she made a surprise performance at the MTV Video Music Awards, performing "DontGetIt" alongside rapper Lil Wayne. On 15 October 2008, Lewis won two awards at the 2008 MOBO Awards: Best Album for Spirit and Best Video for "Bleeding Love". "Forgive Me" was released as Lewis' fifth single in physical CD format and digitally. It was released on 3 November 2008 and reached number 5 in its first week of release in the UK. In addition, Lewis re-released her album Spirit, on 17 November 2008 in Europe. The version contains "Forgive Me", which had been before only on the US and Canadian version, and two other new songs - "Misses Glass" and a cover version of Snow Patrol's "Run". The album later went straight in at number 1 in the UK album charts.

On November 15 2008, Lewis performed her cover of "Run" (originally released by Snow Patrol) on The X Factor, which has been recorded for the deluxe edition of Spirit. Simon Cowell called the performance 'incredible', said how 'proud' he is of her, and said she is 'exactly the same person I met all those years ago', having in fact met her only two years previously. Lewis also performed "Run" on Divas II, which marked the end of Breast Cancer Awareness Month, and was to say a big thank you to its specially-invited audience of fund-raisers and campaigners from three of the UK’s leading Breast Cancer awareness charities: Breakthrough Breast Cancer, Breast Cancer Care and Breast Cancer Campaign. On 23 November 2008 Lewis performed "Better in Time" at the American Music Awards, and later performed "Bleeding Love" on 27 November 2008 at the Bambi Awards, and "Run" at the Royal Variety Show on 17 December 2008. Lewis received three Grammy nominations for the 51st Grammy Awards on December 3, 2008. Her record "Bleeding Love" was nominated for Record of the Year, as well as Best Female Pop Vocal Performance. In addition, Spirit was nominated for Best Pop Vocal Album. Many publications considered her absence from the Best New Artist category a snub.