The group, formed on July 3, 1998, was first named Westside but as the name was already taken by another band, it was changed to Westlife. Boyzonesinger Ronan Keatingwas brought in to co-manage the band with Louis Walsh. The group's original lineup comprised of Nicky Byrne, Kian Egan, Mark Feehily, Shane Filan and Bryan McFadden. Filan is the band's lead vocalist; however, Feehily shares the part of the lead vocals on certain songs as did McFadden before his departure. Some of the members can play musical instruments: Egan (drums, guitar, piano), McFadden (guitar, piano) and Feehily (tin whistle, piano). All of the band members are songwriters, although most of their hits have been composed by external writers.

Westlife's first big break came in 1998 when they opened for Boyzonein Dublin. Later that year, they won the 'Best New Tour Act' award at the Smash Hits Poll Winners Party. In March 1999, the band released their first single, "Swear It Again" which immediately topped the charts in Ireland and in UK for two weeks. Their second single, "If I Let You Go", released in August 1999 along with the highly acclaimed "Flying Without Wings" (their first 'Record of the Year'), released in October the same year, also followed suit. "Flying Without Wings" was also included on the soundtrack of the Warner Brothers film, "Pokemon 2000". The first album, simply titled Westlife, was released in November 1999 and peaked at #2 in the UK. Ironically, the album was the biggest chart dropper in the music history of UK, falling from #3 to #37, and is also the band's only original studio album that never reached #1. In December 1999, the double a side fourth single was released, "I Have A Dream"/"Seasons In The Sun", knocked Cliff Richard's "The Millennium Prayer" off the top spot and earning them the 1999 UK Christmas number-one single. The fifth and last single from the album, "Fool Again", also peaked at #1. Then, the band went on a short UK and Asian tour in support of their debut album before releasing a second album.

"Coast to Coast" was released a year after and was another #1 UK album beating the Spice Girls' Forever album. It becomes the country's 4th biggest selling album of 2000 and is Westlife's biggest selling album to date. The album was preceded by a duet with Mariah Careyon the Phil Collins' classic "Against All Odds (Take a Look at Me Now)" and the original song, "My Love". Both singles reached #1 on the UK charts. With this, Westlife broke an unexpected record of having their first seven singles debut at the top of the UK charts, tying with The Beatles. Outside the UK/Ireland territories, they have released "I Lay My Love on You" and "When You're Looking Like That" and gained chart success.

Westlife released World Of Our Own, their third album, in November 2001. "Uptown Girl", "Queen of My Heart" and "World of Our Own" were released as singles, as of which all peaked at #1 in the UK. "Bop Bop Baby" was also released as a single but it peaked at #5 in the UK Singles Chart.

The band released their eleventh UK #1 single, "Unbreakable", in 2002. Amidst rumors of a split, Westlife released their first greatest hits album in November that same year. Unbreakable - The Greatest Hits Vol. 1 zoomed all the way to #1 in the UK. The release was followed by the double a side single, "Tonight/Miss You Nights", which debuted at #3 in the UK. In September 2003, Westlife released "Hey Whatever", which peaked at #4 on the UK Charts. Their fourth studio album, Turnaround, was then released in November, earning the band another UK #1 album. "Mandy", a cover of the Barry Manilow hit, was released in November 2003. Their version won them their third Record of the Year award in under five years. Their version of Mandy is also considered as the highest single leaper to the top (from #200 to #1) in the UK music history.

On 9 March 2004, Bryan McFadden left the band to spend more time with his family and work on solo projects. On that day, a press conference was held where all the band's members were present and had emotional individual speeches. The four remaining lads thought that it would be the end of Westlife's running. Brian's final public performance as part of Westlife was at Newcastle Upon Tyne's Powerhouse nightclub on 27 February 2004. He subsequently began a solo career.

Less than a month after McFadden's departure, the band kicked off their "Turnaround Tour". They then put out a Rat Pack inspired album, Allow Us to Be Frank, peaking at #3. No singles were released in the UK from this album. In 2005, Westlife returned with their comeback single, "You Raise Me Up", which was taken from their Face To Face album. On 6 November 2005, both the album and the single were at #1 in the UK at the same time at the second week of the single. It was the first time that Westlife have held both top album and single position in the same week. "You Raise Me Up" was awarded as their 4th Record of the Year in UK for 2005. In late 2006, Westlife signed a brand new five album deal with Sony BMG. Their 8th album, The Love Album, was a "love" concept album which consisted of popular love song covers. On November 5, 2007, Westlife released their ninth album, Back Home, which contained nine new original songs along with three cover songs.