Lewis, who is from London, England, began entering singing contests since the age of thirteen. At a party she was introduced to Ben Volpeliere-Pierrot of Curiosity Killed the Cat by her friends who took her to the Metamorphisis recording studios on All Saints Road, London and started doing backing vocals.
It was at the Metamorphosis recording studios where she met Melanie Blatt in 1993 and they proceeded to record. Together with Simone Rainford they formed the group All Saints 126.96.36.199 which was later renamed All Saints. Soon after Rainford left the group and Nicole and Natalie Appleton joined.
Lewis was the most active songwriter of the All Saints and was responsible for writing most of their songs including many hits. In 2001 she won the Ivor Novello Award for best songwriting. After the group broke up, Lewis released her debut solo single "Never Felt Like This Before" in July 2004. It reached no. 8 and is taken from the Top 30 album Open. The next single released was "You" which charted at #56 and which featured a romantic video with an image of her husband in the clouds.
Lewis has also appeared in several films. As a teenager Lewis played three matches for Arsenal Ladies, a successful ladies' football team in London, UK. She returned to football in the 2002 hit movie Bend It Like Beckham in the role of the team captain and can be seen playing football in several training and match scenes. that same year, she played a role in Hideous Man.
On August 21, 2004 Lewis married dancer Christian "Storm" Horsfall. In November 2004 Shaznay was involved in the Band Aid 20 re-recording of "Do They Know It's Christmas." Chasney gave birth to her their first child, a son named Tyler-Xaine, in February 2006.
One month earlier, it was announced that All Saints had reformed with a new recording contract and would release a new album, Studio 1 in November 2006. She performed with All Saints on live television for the first time since reforming on the UK light entertainment program, Ant & Dec's Saturday Night Takeaway on October 21, 2006. All Saints' comeback began well, with their single, "Rock Steady" reaching no. 3 in the UK charts. They quickly followed this with the release of their comeback album Studio 1. The album peaked at no. 40 in the UK album charts.
Parlophone Records then released their second single "Chick Fit," but this failed to reach the top 200. All Saints then parted with the record label.
In January 2008 Lewis featured on the Wideboys track, "Daddy O". The single was released on 5 May 2008.
Lewis is currently writing songs for other projects. She's credited as songwriter in Westlife's track "Reach Out," which was featured on their 2009 album Where We Are, and also co-wrote the CocknBullKid track "Distractions" from her 2011 album Adulthood In November 2009, Lewis and her husband had their second child, a daughter named Tigerlily.