At the age of 13, Lee played the leading role in the West End production of Oliver! At the London Palladium Theatre alongside Jonathan Pryce's and George Layton's Fagin.
The group rose to fame by starring in their own BBC television series, Miami 7, in 1999 and have been widely referred to as inspiring a new generation of teenage TV musicals including High School Musical and Glee. Over the five years they were together, S Club 7 had four UK No.1 singles, one UK No.1 album, and a string of hits throughout Europe, including a top-ten single in the United States, Asia, Latin America and Africa. They recorded a total of four studio albums, released eleven singles and went on to sell over fourteen million albums worldwide. Their first album, S Club, had a strong 1990s pop sound, similar to many artists of the time. Through the course of their career, however, their musical approach changed to a more dance and R&B sound which is heard most notably in their final album, Seeing Double. S Club 7 won two Brit Awards — in 2000 for British breakthrough act and 2002 for best British single. In 2001 the group earned the Record of the Year award. Group member Paul Cattermole departed the group in 2002, and the group changed its name from S Club 7 to simply S Club. On 21 April 2003, during a live onstage performance, S Club announced that they were to disband.
In 2004 Lee played the role of Marius in Les Misérables at the Palace Theatre in London and continued the position following the show's move to the smaller Queen's Theatre. In 2005 Lee toured the UK with the new musical, Love Shack, featuring disco classics and hit dance songs from the charts, starring alongside Steps' Faye Tozer and Hear'Say's Noel Sullivan. In August 2006 Lee starred in The Heather Brothers' production of Teen Scream at the Assembly Rooms at the Edinburgh Festival Fringe. He returned to the London stage in 2010 in the musical Tomorrow Morning. Lee worked on some other musical projects before returning to the stage, and also starred in BBC's "The Sound of Musicals". In 2008 he recorded a song for the CD Act One – Songs From The Musicals Of Alexander S. Bermange, an album of 20 new recordings by 26 West End stars. It released in November 2008 on Dress Circle Records. In 2008 Lee voiced the part of good-looking, bike-riding Max, in the British Disney Channel cartoon series Famous 5: On the Case. Over Christmas and New Year's 2008–09, he appeared at The White Rock Theatre, Hastings, performing the leading role in the pantomime Peter Pan. Both Lee and the show were well received. Later in 2009, Lee appeared as Billy Fury in Nick Moran’s new film Telstar. On 22 May 2010, he appeared in BBC One's Casualty as Craig, who unexpectedly gets diagnosed with motor neurone disease. He played John in the internationally acclaimed musical Tomorrow Morning by Laurence Mark Wythe at the Landor Theatre in London. Over the following Christmas and New Year's, Lee appeared at the New Victoria Theatre, Woking as Prince Charming in Cinderella. In October 2012 Lee announced, via Twitter, that he had recorded his debut solo album, Fallen Angel. The first track on the album, "My Father's Son", was premiered on 4 November by BBC Radio 2's Paul O'Grady. The album was released on 4 March 2013. Lee took on the part of Frankie Valli in Jersey Boys at the Prince Edward Theatre, London, from 15 March 2011 until 7 March 2014. At the end of 2012, while maintaining his Jersey Boys commitments, Lee took the lead role in Aladdin at The O2 in London and acted alongside Paul O'Grady, who revived his alter-ego, Lily Savage. In August 2016 Lee appeared at the Lytham Festival's "West End Proms". In March 2017 Lee appeared on Let's Sing and Dance for Comic Relief as part of Boys Allowed with Gareth Gates. Duncan James, Ritchie Neville and Ben Ofoedu. In March 2018 he appeared on ITV's Dinner Date.