Lewis commented on the musical style of her debut album describing the album as "classic songs with a contemporary edge", with R&B and "fresh pop" styles, ballads and "soulful up-tempo numbers". It has an American style, with some electronic 1980s sounds. However, the songs are not beat-driven or following the latest trends but can be performed acoustically. Lewis's second album, "Echo", mixed pop and R&B; during an interview with Variety magazine, she said on the album's sound that she "wanted the album to have a little bit more of a live feel to it, with a little more live instrumentation." In February 2011, talking about her third studio album "Glassheart", Lewis said that the album would be more experimental than Spirit and Echo by embodying a "different" yet "classic" sound. She added, "I'm really, really excited about it. I'm working with some new producers, some up and coming people and it's going to be kind of a different sound – but still classic". Lewis described it as "energetic, deep, [and] unique". She also commented that it would have a darker tone and that she would be putting her "heart into [her] lyrics".

Lewis was born on 3 April 1985 in Islington, London, England, to a Welsh mother and a father of Guyanese descent. She has an older half-brother, Bradley, and a younger brother, Kyle. Noticing Lewis's passion for singing, her parents enrolled her at the Sylvia Young Theatre School and later at the Italia Conti Academy of Theatre Arts, the Ravenscourt Theatre School and the BRIT School for Performing Arts and Technology until they could no longer afford to do so, making cutbacks when necessary to help their daughter achieve her aspiration of being a singer. Lewis wrote her first song at the age of 12; she initially trained in opera before moving into jazz and blues and eventually into popular and contemporary music.

Lewis recorded a demo album, called Twilight, in collaboration with Spiral Music, a production company based in Fulham, but it failed to secure her an album deal with any record companies. Although the album was never released commercially, Lewis performed a couple of the tracks live on BBC Radio 1 music station in 2004. Another demo album was recorded under license from UEG Entertainment, Best Kept Secret, which have reportedly claimed to have spent £70,000 trying to launch Lewis's career with no success. A track from the album, "Private Party", became a hit on the underground urban music scene in London in 2005. Speaking about her struggle to secure a record contract, Lewis said, "I tried to secure a record deal by doing things my own way. I worked very hard but I never managed to land a contract. That's why I decided to audition for The X Factor. It's programmes like these which provide the best place to showcase fresh new talent."

Lewis's winner's single, a cover of Kelly Clarkson's song "A Moment Like This", was released on 17 December 2006. In the United Kingdom, it broke a world record after being digitally downloaded more than 50,000 times in less than 30 minutes. The song became the 2006 UK Christmas number-one single, selling 571,000 copies in its first week and selling more than the Top-40 combined. It spent four consecutive weeks atop the UK Singles Chart and was number one in Ireland for six weeks. Despite being released on 17 December, "A Moment Like This" became the second best selling single in the United Kingdom in 2006, and it has been certified platinum by the British Phonographic Industry (BPI), denoting shipments of over 600,000 copies. As of December 2014, Lewis's version has sold 900,000 copies in the United Kingdom.

Lewis's second single, "Bleeding Love", produced by Ryan Tedder and written by Tedder and Jesse McCartney, was released in October 2007 in the UK, where it sold 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, and in the Irish Singles Chart, it remained at number one for eight weeks. It reached number one in the singles charts of New Zealand, Australia, France, Germany, Norway, Switzerland, Belgium, The Netherlands, Austria, Canada and the United States. "Bleeding Love" won The Record of the Year in December 2007. In February 2008, "Bleeding Love" entered the Billboard Hot 100 at number 85 and then went on to peak at number one for four non-consecutive weeks. The song became the first track by a UK female to reach number one since Kim Wilde's "You Keep Me Hangin' On" in 1987. Lewis's 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 single reached a peak of number two in the UK singles chart selling over 40,000 copies in its first week of physical release. "Better in Time" was released as Lewis's second single in the US, where it peaked at number 11 in the Billboard Hot 100. "Forgive Me" was released as Lewis's fifth single in November 2008; it reached number five in the UK. "Run" was released as a download-only single in the UK, reaching number one and becoming the UK's fastest-selling download-only single, with 69,244 copies sold in two days. Lewis's last single from Spirit, "I Will Be", was released in January 2009, only in North America.

In October 2008, The Times reported that Lewis had turned down a seven-figure sum from Mohamed Al Fayed to open a Harrods sale. Lewis commented that she turned down the deal on the grounds that Harrods is the only UK department store that continues to stock clothing made from animal fur: "It wasn't a million pounds that I was offered, as the papers reported, but even if it had been, I still would have turned it down". "I got a lot of flak for that. There were people who said I should have done it and given the money to charity, but that would have been such a contradiction." She announced in October 2008 that she was in the "bargaining period" of launching her own ethical line of accessories through Topshop and that she was in the late stages of releasing her own perfume in Europe. Her perfume, Leona Lewis, was launched by LR in July 2009. In 2010, she set up a fashion company with her then-boyfriend, Lou Al-Chamaa, called LOA Clothing Ltd, which was dissolved in 2012.

Lewis signed a book deal in January 2009 to release an illustrated autobiography in October 2009. The book, Dreams, contains mostly pictures taken by Photographer Dean Freeman. At a book signing for Dreams on 14 October 2009, at the Piccadilly branch of Waterstone's book store in central London, Lewis was assaulted by a 29-year-old man from south London who began punching her in the head. He was arrested at the scene; The assailant was sectioned under the Mental Health Act 2007 and was charged with Common assault. He admitted to the offence and was hospitalised for an indeterminate period.

Lewis's first tour, titled The Labyrinth supporting Spirit and Echo, started in May 2010, with a theme based on the film Labyrinth. Lewis was scheduled to tour North America from July to August 2010 supporting Christina Aguilera's Bionic Tour, but Aguilera cancelled the tour, leaving Lewis's plans unknown. A DVD of the tour, along with a ten-track CD, was released with the title The Labyrinth Tour Live from The O2 on 29 November 2010.

In 2011, Lewis was a guest judge on Platinum Hit, and in May 2012, she returned to The X Factor as a guest judge during auditions in London for the ninth series. In March 2013, Lewis was announced as the new brand Activist for The Body Shop. She has also shown support for Little Kids Rock, a nonprofit that works to restore and revitalize music education in disadvantaged US public schools by donating items for auction to raise money for the organization. Lewis attended and supported the annual H.E.A.R.T Organization in 2017, a violence intervention programme.

Lewis lives in Hackney, East London. Lewis has been a vegetarian since she was 12, and she has been vegan since September 2012. She won PETA's Sexiest Vegetarian, along with Red Hot Chili Peppers frontman Anthony Kiedis, in 2008 and Europe's Sexiest Vegetarian in 2009, along with actor Scott Maslen. She also was named PETA's Person of the Year for 2008. Lewis is a supporter of World Animal Protection and is a patron of the Hopefield Animal Sanctuary in Brentwood, Essex.

In December 2013, Lewis told Billboard magazine, "Earlier in the year, I was getting in with a few different producers, I actually got in with DJ Poet, who is an amazing Producer. We did a couple of songs together, which has been really exciting. I will probably get back in and work with him again".

In September 2014, in an open letter to fans, Lewis revealed that she felt extremely depressed around the time of her departure from Syco. She later emphasised that it was not clinical depression from which she had been suffering. She credited her professional separation from Simon Cowell and her practice of meditation with helping her "still the madness".

On 11 September 2015, following the album's release, Lewis announced her third live tour, with an initial leg of fourteen UK dates. On the tour, Lewis visited major cities, including Birmingham, London, Cardiff and Glasgow. On 12 December, the singer performed at The X Factor UK final, alongside fellow X Factor UK winner Ben Haenow. On the show the duet sang Lewis's song "Run" and Haenow's "Slamming Doors". The same month, Lewis performed a cover of "Girl Crush" with Adam Lambert. In May 2016, the performance earned the pair a CMT Music Award nomination.

On 18 March 2016, Lewis headlined Cirque du Soleil's One Night for One Drop performance in Las Vegas. On 26 May 2016, it was announced that Lewis would make her Broadway debut, as Grizabella, in the July 2016 revival of Andrew Lloyd Webber's musical Cats. She left the role, after almost four months of playing the character, on 9 October 2016. In June 2016, after two years with the label, it was revealed that Lewis and Island Records parted ways because of "something that didn't click". Later that month, she released the song "(We All Are) Looking for Home", a charity single created in collaboration with Diane Warren for the Vanderpump Dog Foundation's public Service announcement, in opposition to the Yulin Dog Meat Festival.