Geraldine Estelle Halliwell was born at Watford General Hospital, Hertfordshire, to Laurence Francis Halliwell (1922–1993), who was of English and Swedish descent, and his Spanish wife Ana María (née Hidalgo), who is a native of Huesca. Halliwell grew up on a council estate in North Watford. She was educated at Watford Grammar School for Girls and Camden School for Girls.

Before starting her music career, Halliwell had worked as a nightclub Dancer in Majorca, as a presenter on the Turkish version of Let's Make a Deal, and as a glamour model. At the age of 19, she appeared in The Sun as a Page 3 girl. Following her rise to fame with the Spice Girls, nude photos of Halliwell were republished in a number of magazines in 1992 and 1996; these included Playboy and Penthouse.

In 1994, Halliwell, along with Melanie Chisholm, Melanie Brown, and Victoria Beckham, responded to an advertisement in The Stage magazine. Around 400 women answered the ad and auditioned at Dance Works studios. After a couple of initial line-up changes, Halliwell, Chisholm, Brown, and Beckham (née Adams) eventually became the members of the group, along with Emma Bunton. The group felt insecure about the lack of a contract and were frustrated by the direction in which Heart Management was steering them, and in October 1994, armed with a catalogue of demos and dance routines, they began touring management agencies. They persuaded Bob Herbert to set up a showcase performance for the group in front of industry Writers, producers, and artists and repertoire men in December 1994 at the Nomis Studios in Shepherd's Bush, where they received an "overwhelmingly positive" reaction. Due to the large interest in the group, the Herberts quickly set about creating a binding contract for the group. Encouraged by the reaction they had received at the Nomis showcase, all five members delayed signing contracts on the legal advice from, amongst others, Adams' father Anthony Adams. In March 1995, because of the group's frustration at their management's unwillingness to Listen to their visions and ideas, they parted with Heart Management.

The group began a relationship with Simon Fuller of 19 Entertainment and finally signed with him in March 1995. During the summer of that year, the group toured record labels in London and Los Angeles with Fuller and finally signed a deal with Virgin Records in September 1995. From this point on, up to summer 1996, the group continued to write and record tracks for their debut album while extensively touring the West Coast of the United States, where they had signed a publishing deal with Windswept Pacific. On 7 June 1996, the Spice Girls released their debut single, "Wannabe", in the United Kingdom. In the weeks leading up to the release, the video for "Wannabe" got a trial airing on The Box music channel. The song proved to be a global hit, reaching number one in 37 countries, becoming the best-selling single by an all-female group of all time. It was followed by nine further number-one singles from their albums Spice, Spiceworld, and Forever. Each member of the group received a nickname from the media. Halliwell was named "Ginger Spice". Other successful releases followed, including "Say You'll Be There" and "2 Become 1" from Spice, and "Spice Up Your Life", "Too Much", and "Viva Forever" from Spiceworld.

In the run-up to the 1997 UK General Election, Halliwell declared: "I saw a lot of what Mrs Thatcher did. She was definitely the original Spice Girl rising from the greengrocer's daughter to Prime Minister." She claimed that her background was deeply rooted in support for the Conservative Party and that Thatcher was the pioneer of "Girl Power" and the spiritual sixth member of the Spice Girls. She claimed in 1996 that Tony Blair was "not a safe pair of hands for the economy". By the 2001 general election Halliwell had switched her support to Tony Blair and the Labour Party, making a cameo appearance in one of their party election broadcasts.

On 31 May 1998, Halliwell announced that she had left the Spice Girls due to depression and differences between the group. The first official confirmation was an announcement to the media by her solicitor on 31 May. Her action aroused controversy, her former group being due to embark on a North American tour, which they eventually completed without her. Although she had already left the group, the Spice Girls released "Viva Forever", the final music video to feature Halliwell's likeness, plus a one-off supergroup called England United for the official England FC song (Jo Whiley introduced the band saying "...plus Geri as a substitute"). After she left, the other girls co-wrote a few songs about her, which appeared on their album Forever: "Goodbye", "Tell Me Why", and "Let Love Lead the Way". The group is among the best-selling girl groups of all time, selling over 75 million albums.

In 1999, Halliwell became a Goodwill Ambassador for the United Nations Population Fund (UNFPA). In 2000 Halliwell appeared in the two-part documentary series Geri's World Walkabout for the BBC, which followed her work with the UN and other travels. That same year, she gave the opening address at the UN Youth Summit. Halliwell picked up further UN work in 2006, by visiting Zambia from 14–16 November, to promote greater international awareness of the urgent need to reduce maternal death and halt the spread of HIV/AIDS. In September 2009, Halliwell, in her role as a Goodwill Ambassador of UNFPA, visited Nepal to help launch a national campaign to stop violence against women.

In 2001, Halliwell followed up with her second album, Scream If You Wanna Go Faster, the album peaked at number five in the United Kingdom, and sold 200,000 copies there and was certified gold. It included her cover version of the Weather Girls' 1983 hit, "It's Raining Men", was also used on the Bridget Jones's Diary film Soundtrack, and the video game, DDRMAX2 Dance Dance Revolution 7thMix. The song was released as the lead single from the album. "It's Raining Men" became a major hit worldwide, it peaked at number one in the United Kingdom and peaked in the top 10 in over 27 countries, becoming Halliwell's biggest hit to date. The song won her the International Song of the Year award at the 2002 NRJ Music Awards. The song originally been added to the album at the last minute, and another song, "Feels Like Sex", had already been slated as the lead single. The singles that followed, "Scream If You Wanna Go Faster" and "Calling", reached number eight and number seven in the United Kingdom, respectively. Halliwell released a special French edition of "Calling", titled "Au Nom de L'amour". Scream If You Wanna Go Faster achieved lower success outside the United Kingdom than Schizophonic and was not released in the United States. In 2002, Halliwell was once again nominated at the BRIT Awards, this time for Best British Female Solo Artist and Best British Single for "It's Raining Men". Scream If You Wanna Go Faster has sold a little less than 1,300,000 copies worldwide. In 2002, Halliwell was featured alongside Pete Waterman and Louis Walsh as a judge on the television series Popstars: The Rivals, which created Girls Aloud.

In late 2004, Halliwell made a return to music with the single "Ride It", which reached number four in the United Kingdom and number one on the dance charts. However, several months elapsed before another single was released, during which time she was apparently instructed to record some new tracks for the as yet unreleased album by her record company, which was unhappy with the setlist. Halliwell planned her first solo tour of the United Kingdom and Ireland, but due to lack of ticket sales, compounded by the label's pressure on Halliwell to record additional songs, led to the cancellation of the tour. Eventually, a new single, "Desire", was released on 30 May 2005, reaching number 22 in the UK Singles Chart and number one on the UK Dance Charts. Released shortly after, the source album, Passion, similarly received little attention from the public or critics, and stalled at number 41 in the British charts. Halliwell's recording contract with EMI was subsequently not renewed, and in subsequent interviews, Halliwell stated that she was not interested in recording another album at that time and was content with writing children's books and being a mother.

Despite being a "girl power" icon, Halliwell publicly denounced being labelled as a feminist in a 2007 interview with The Guardian, for which she received wide-scale media attention: "For me, feminism is bra-burning lesbianism. It's very unglamorous...We need to see a celebration of our femininity and softness."

The character Princess Posh Vattoria, a caricature of Victoria Beckham, was featured in early drafts but did not appear in the book series. Other characters are said by Halliwell to be loosely based on Gordon Ramsay, George Michael, Marilyn Monroe, Vincent van Gogh, Wayne Rooney, and the character Justin Suarez from the TV series Ugly Betty. According to the official site, the book sold more than 250,000 copies in its first five months, making its author Halliwell 2008's most successful female Celebrity children's author.

In May 1999, Halliwell appeared in a 90-minute documentary Geri for the British television channel Channel 4 by Molly Dineen. The documentary was a ratings success with 4.5 million viewers—almost one-fifth of available viewers—and Channel 4 aired it again one month later. She has also released two yoga DVDs with her yoga Teacher Katy Appleton, Geri Yoga and Geri Body Yoga. Halliwell appeared as a judge on the UK talent show Popstars: The Rivals and on the United States-produced reality programme All American Girl and as a guest reporter on celebrity-based series Extra. She has also made appearances in the television series Sex and the City and in the 2004 film Fat Slags, based on characters from Viz magazine. In 2004, Halliwell appeared on Channel Five as one of the hosts of the Party in the Park event for the Prince's Trust, the presenter and main performer of the Tickled Pink Girls' night in Live! event and an appearance in a documentary There's Something About Geri. Halliwell appeared in the film Crank: High Voltage alongside her friend, actor John Damon, and on the BBC One's The One Show on 7 May 2009.

Halliwell acted as a guest judge on UK talent show The X Factor in 2010, and 2012 and she was a judge on season seven of Australia's Got Talent. She guest presented an episode of The One Show in October 2015. Halliwell appeared on episode three of the 2016 series of Sport Relief Bake Off. In March 2017, Halliwell presented the BBC Two documentary Geri's 90s: My Drive to Freedom as part of the BBC Music: My Generation series. She sat in for Zoë Ball on BBC Radio 2 in July 2017. She guest presented an episode of This Morning in August 2017 with Rylan Clark-Neal.

Halliwell has spoken out about her experiences of bulimia, saying that she came close to death, weighing just 7 stone (98 lb; 44 kg), and was advised by Robbie Williams to seek medical help. In 2011, she spoke of being comfortable with her body and credits her newly healthy relationship with food to motherhood.

In February 2012, Halliwell announced that the fourth album was being mastered. In August 2012, she reunited with the Spice Girls to perform as a quintet for the last time at the closing ceremony of the Olympic Games in London. This was the last Spice Girls concert to feature Victoria Beckham and Mel C. In 2010, Halliwell stood in for Dannii Minogue as a guest judge on The X Factor at the Glasgow auditions alongside Simon Cowell, Louis Walsh, and Cheryl Cole, and she returned again in 2012 as a guest judge at the Liverpool auditions alongside Walsh, Gary Barlow, and Tulisa Contostavlos. In October 2012, Halliwell made her first solo performance in seven years at the Breast Cancer Care, debuting a new track called "Phenomenal Woman". In January 2013, Halliwell claimed that she had decided to shelve the album she had finished to continue writing and recording, moving in a different style direction. In April 2013, the Nine Network announced that she would become the fourth judge on Australia's Got Talent.

On 5 September 2013, it was confirmed by Channel 9's NRL Footy Show (one of the longest-running shows in Australia) that Halliwell would be performing a world exclusive of "Half of Me", her first solo single in nearly eight years, at its grand final show. The performance took place on 3 October 2013, exactly one year after her last exhibition with new material at the Breast Cancer Care Show. On 12 September 2013, it was announced that the song "Half of Me" would be released exclusively in Australia, as part of a new deal with Sony Music Australia. The single was released on 25 October 2013 and reached number 281 in the Australian charts. Halliwell then took to her official blog to write about her upsetting experience back in the music Business and hinted of possibly retiring for good. On the grand final of Australia's Got Talent, she performed an acoustic version of the Spice Girls' hit single "Wannabe".

Halliwell gave birth to her daughter, Bluebell Madonna, in May 2006. The child's father is Screenwriter Sacha Gervasi with whom Halliwell was in a relationship in 2005; Victoria Beckham and Emma Bunton are godmothers. Halliwell began dating Christian Horner, the team principal of the Red Bull Racing Formula One team in February 2014. They announced their engagement on 11 November 2014, and the couple were married on 15 May 2015 at St Mary's Church in Woburn, Bedfordshire.

On 25 October 2015, the Mail on Sunday reported that Halliwell had been in talks with the Department for Education with a view to establishing a free school in north London, scheduled to open in 2018 and specialising in the arts and Business. Halliwell told the newspaper: "I believe in education. It’s an empowering fundamental human right that everyone deserves. Education is a foundation for life." The Department subsequently confirmed that it had spoken with Halliwell, but that discussions were at an early stage.

On 22 November 2016, 13 songs from Halliwell's fourth album, reportedly titled Man on the Mountain, were leaked onto the internet, including the previously confirmed tracks: "Love and Light", "Phenomenal Woman", and "Sheriff".

In October 2016, Halliwell revealed that Horner and she were expecting their first child together. Halliwell gave birth to her second child, a son, named Montague George Hector Horner, on 21 January 2017. Halliwell is stepmother to Horner's daughter, Olivia, from his previous relationship.