Halliwell was born to Laurence Francis Halliwell, who was of English/Swedish descent, and his wife Ana María (née Hidalgo), who was of Spanish descent from Huesca, Spain and who grew up in Watford, Hertfordshire. For a brief time, Halliwell's mother tried to bring her up as a Jehovah's Witness.

Halliwell took her A-Levels at Camden School for Girls having taken her GCSEs at Watford Girls Grammar School. Before starting her music career, Halliwell had worked as a nightclub dancer in Majorca (Spain), a model and presenter on the Turkish version of Let's Make a Deal and as a glamour model.

Following her rise to fame with the Spice Girls, nude photos of Halliwell were re-published in a number of top-shelf magazines in the UK. During the documentary Spice Girls: Giving You Everything broadcast on UK television on 31 December 2007, Halliwell repeated her claim to have only done 'harmless, topless shots', despite extensive evidence to the contrary, including pictures in her own autobiography.

Geri first found fame as one of the five members of the girl group the Spice Girls, and was dubbed Sexy Spice for her seductive outfits and stage costumes. However, she preferred being called Ginger Spice stating, "I always thought of myself as clumsy, not sexy." Of her over-the-top outfits, she is mostly recognized for her famous Union Jack dress that she wore for the 1997 Brit Awards.

Halliwell became renowned for her independent and feminist attitude. The group joined the most successful musical artists of the 1990s, selling over thirty-five million albums with the studio releases Spice and Spiceworld. Their debut single "Wannabe" became the first of a string of number-ones in the United Kingdom (nine in total) and reached the top position in 41 countries around the globe, including Australia, Canada and the United States.

Other successful releases followed, including "Say You'll Be There" and "2 Become 1" from Spice, and "Spice Up Your Life", "Too Much" and "Stop" from Spiceworld. As a songwriter, Geri - as part of the Spice Girls - set the record for the shortest time to achieve 6 no. 1 UK singles, taking 1 year 5 months from Wannabe's first week at no. 1 to the ascent of Too Much. This outpaced Lennon/McCartney, who took 1 year 6 months (From Me to You through I Feel Fine).

On May 30, 1998 Halliwell 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 band 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. After she left, the other girls wrote a few songs about her which appeared on their album Forever, notably "Goodbye" (although it is said that Geri had helped write this song before her departure, but since the song was released after her departure, most people see it as a tribute to Geri) and "Let Love Lead the Way."