Prior to becoming successful, Kian, Mark and Shane along with fellow Sligonians Derrick Lacey, Graham Keighron and Michael Garrett, were part of a six-member pop vocal group called IOYOU. Managed by choreographer Mary McDonagh along with two other informal managers, they released a single titled "Together Girl Forever". Louis Walsh, the manager of the 1990s boy-band Boyzone, was contacted by Shane's mother and came to know about the group. However, IOYOU did not meet Simon Cowell's approval for a record deal with BMG, and thus changes were needed. Three members of IOYOU were told they would not be part of the new group, and auditions were held in Dublin where Nicky Byrne and Brian McFadden were recruited. The new group, formed on 3 July 1998, was re-named Westside but as the name was already taken by another band, it was changed to Westlife. Also changing the spelling of Brian's name to Bryan. Boyzone singer Ronan Keating was brought in to co-manage the group with Walsh. Westlife then released an EP titled Swear It Again, though only in Asia.

Westlife's first big break came in 1998 when they opened for the Boyzone and Backstreet Boys' concerts in Dublin. Later that year, they won the 'Best New Tour Act' award at the Smash Hits Poll Winners Party. In March 1999, the group 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, "Pokémon 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 group'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 group 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 became 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 Carey on the Phil Collins' classic "Against All Odds (Take a Look at Me Now)" and the original song, "My Love" (the song that bagged their second 'Record of the Year' award). 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. However, in December 2000, they failed to secure their eighth chart-topper in a row with their UK and Ireland only single "What Makes A Man" when kid's favourite, Bob The Builder's "Can We Fix It", was crowned as the Christmas number-one single of that year. 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. In 2001, the boys launched their first world tour, "Where Dreams Come True Tour", unofficially nicknamed "The No Stools Tour" due to the group's reputation of performing while perched on stools.

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. In 2002, Westlife went on their second world tour, the World Of Our Own Tour (In The Round)

The group released their eleventh UK #1 single, "Unbreakable", in 2002. Amidst rumours of a split, Westlife released their first greatest hits album in November that same year entitled, 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. At this era, Because Films Inspire... made a TV Documentary titled as "Wild Westlife" directed by Iain MacDonald and starred by the group itself. It reveals how was their musical life on a daily basis and when on tour. In 2003, Westlife went on to their third world tour, The Greatest Hits Tour, shrugging off rumours of a split.

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 group 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. "Obvious" was released as the final single from the album, charting at #3. On 9 March 2004, just three weeks prior to embarking on their fourth world tour, Bryan McFadden left the group to spend more time with his family and work on solo projects. On that day, a press conference was held where all the group's members were present and gave emotional individual speeches. Bryan'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, deciding to revert the spelling of his first name back to its original 'Brian', he released his first solo single, "Real To Me" which entered the UK chart at #1 and soon after, he released his first solo album, "Irish Son" under Sony Music. He went on to release more singles with moderate success.

Less than a month after McFadden's departure, the group kicked off their "Turnaround Tour". A live version from the said tour of "Flying Without Wings" was released as an official UK download, earning them the title of the very first UK Downloads #1. 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. "Ain't That A Kick In The Head", accompanied with a music video, was released as a physical single in other European countries. "Smile" and "Fly Me To The Moon", with music videos as well, were released as digital downloads only. Prior to the album release, Westlife scouted for "the perfect fan" to help promote their album from their one-season special show "She's The One", presented by Kate Thornton. After X Factor-style auditions, they found Joanne Hindley, who recorded "The Way You Look Tonight" with the group. To mark this special collaboration, a special programme was televised, showing auditions and live performances. Performers included Joanne, Westlife members' fathers and glamour model Katie Price, Il Divo and Kevin Spacey. Westlife continued to tour Europe as part of their "The Number Ones Tour", before taking a four-month break from work. On 4 May 2005, Westlife lost their battle for European trademark from the German trademark "West". 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 fourth Record Of The Year in UK for 2005. In December that year, the group released "When You Tell Me That You Love Me", a duet with Diana Ross, as the second single and it debuted at its peak position of #2. Westlife then released a third single, "Amazing", which debuted at #4 and marked as the group's lowest-selling single yet. After that, Westlife embarked on the "Face To Face Tour", travelling extensively to the UK, Ireland, Australia and Asia. This tour marked the first time that Westlife have travelled to mainland China for a concert.

In late 2006, Westlife signed a brand new five-album deal with Sony BMG. Their eighth album, The Love Album, was a "love" concept album which consisted of popular love song covers. The album outsold other compilation albums by Oasis, The Beatles, and U2 in its first week of release and went straight to #1. The first single from The Love Album was a cover version of Bette Midler's classic, "The Rose", which became their 14th UK #1 single. This made Westlife the third act (along with Cliff Richard) in the UK to have the most #1 singles, tailing behind Elvis Presley (21) and The Beatles (17). Westlife then kicked off their eighth world tour, "The Love Tour", in Perth, Australia. The group then went on to other Australian cities before moving on to South Africa, the UK and Ireland. Westlife staged their last major concert of 2007 at Bitts Park in Carlisle, England.

On 5 November 2007, Westlife released their ninth album, Back Home, which contained nine new original songs along with three cover songs. The album debuted at #1 on the UK Music Charts and it was also 2007's fifth biggest selling album in the UK. The first single released from the album was a cover of Michael Bublé's "Home", which peaked at #3 in the UK. "I'm Already There", not released as a single, managed to chart in the UK based on downloads alone following a performance on an The X Factor episode. On 15 December 2007, they had a two-hour show called The Westlife Show where they performed 10 of their songs, some voted by the fans through online and some from Back Home. It was hosted by Holly Willoughby. This time, Westlife together with Tesco brought out "The Definitive Westlife Quiz" that asked about the whole history about the boys. It was participated by thousands of fans worldwide. Months later, "Us Against The World" was announced and released as their second single in UK and Ireland. It peaked at #8 and is the group's lowest charting single to date. Before the release of the second single, they embarked on the Back Home Tour on 2008 February 25. This tour marked the first time that the group have traveled and performed to New Zealand, performing four sold-out shows in Auckland, Wellington, New Plymouth and Christchurch. Meanwhile, "Something Right" was released as the second single and "Us Against The World" became the third single in Europe and the Asia Pacific region. Both songs performed well on several music charts.

To mark their tenth year in music, Westlife staged a special 10 Years Of Westlife sold-out concert at the world's thirty-third and Europe's fourth biggest stadium, Croke Park, on 1 June 2008. With a capacity of 82 300, it was the boy's biggest show to date. Shayne Ward filled in as a support act for the concert, which Kian described to be of a "pop extravaganza". Shane confirmed that a corresponding live concert DVD will be released. The group announced that they will be on hiatus for a year after their Back Home Tour and that there would not be an album release in 2008 as they would be spending more time on the production of their tenth album. On 16 June 2008, HarperCollins UK will release Westlife's autobiography, 'Westlife - Our Story', as part of their 10th year celebration. The book was said to be written by each group member for three months. It contains the life they have for the past ten years in the business. It features also some of the private pictures from the lads. As promised, the group's official website confirmed on 27 September 2008 the release of a DVD on 24 November 2008 entitled 10 Years Of Westlife - Live at Croke Park Stadium which went straight to #1 on UK, Ireland, South African, Hong Kong and New Zealand Music DVD charts.

As the group ended their successful Back Home tour, Louis Walsh announced in the show Xpose that 1 July 2008 is the official start of the longest hiatus of the group so far. He said that it will be a one-year break, from that day up to 1 July 2009. On 13 December 2008, while on a break, Westlife made an unexpected appearance on X Factor Final Performance Night where they performed "Flying Without Wings" with JLS. After the presentation, Shane and Nicky were interviewed on Xtra Factor with Ronan Keating and Stephen Gately of Boyzone. As JLS performed also "I'm Already There", Westlife's version of the song re-entered the UK Singles Chart at #63 while a new entry on Ireland Singles Chart at #47 due to extensive downloads only.

Before Christmas 2008, they wrote a letter to their website saying that 2009 was going to be the most exciting year yet for Westlife. With that, the fans of Westlife strongly speculated that one of the reasons is because of their possible collaborations with Leona Lewis and Britney Spears. While on a hiatus, Westlife was reported to have a closed deal for their upcoming 70-concert tour worth 10 million Euros by the end of 2009.

On the last week of January 2009, a DVD entitled The Karaoke Collection was released. It contains some of their music video hits. In 27 February 2009 issue of Herald Ireland, Louis revealed that Simon already picked three new songs which he reckons to be instant hits. Nicky spread the word last 8 March 2009 that Westlife will come back to music charts when they start recording in June 2009 when he received his VIP Style Awards as the Most Stylish Man on X-pose, TV3. On 18 March 2009, they won the Best Irish Pop Act on 2009 Meteor Awards for the ninth consecutive time. On 28 May 2009, they re-released Westlife - Our story autobiography, adding their Croke Park experience and first time in a paperback edition.

Walsh has revealed that after a year's break with their friends and family Westlife will return to hit the charts once more in June when they work on their tenth studio album. Walsh also said that while he met up with Simon Cowell they jointly managed to mastermind Westlife's comeback. At the time three songs had already been chosen which he believes will be big hits when they are released. Westlife will be revealing their 2010 tour dates with two new dates in Croke Park on their minds. They have currently set one date at Croke park to be June 5. In total the boys plans to play 70 concerts coming towards the end of the year including 8 dates at Dublin's 02 arena and other huge venues across Ireland, the United Kingdom and other countries. Shane told the Irish Mirror that they want to make the next album one of their best.

The recording process of their tenth album started on 1 July 2009. Nicky said that there will be many first-time producers and writers for the upcoming album. He also wanted to have collaboration works with Chris Martin of Coldplay who happens to be a big Westlife fan. Ryan Tedder of the group OneRepublic and a well-known writer/producer have also done songs for them called "Shadows" (co-written with AJ McLean of the Backstreet Boys) and "Beautiful", which may appear on their forthcoming album. Another song has been found for them entitled "Leaving". Shaznay Lewis were once reported to work with the pop group. Before the proper album release, "As Love Is My Witness" recorded by the boys and "Now Can't Let Go", a Westlife rumored demo song by Wayne Hector surfaced the internet.

The new album, Where We Are, was released on 30 November 2009 in the UK and peaked at #2 on both Irish and UK Albums Chart. The new single "What About Now", was released a few weeks earlier on 23 October 2009, with digital downloads being available the day before. The said single peaked at# 2 on both Irish and UK Singles Chart. Daughtry's original version of the song gained exposure on the audition rounds of The X Factor prior to Westlife's release. The group is set to collaborate for the second time with Mariah Carey and is going to be Mariah's next single, the song is called "Call Me Now" and will be exclusive to the UK. They will be part of the Haiti charity single, "Everybody Hurts", an R.E.M. cover to be organized by Simon Cowell. The second single is "Reach Out" expected to be released on March/April 2010. Sony Music said that the news about the second single was just a rumour.

Shane revealed on the Paul O' Grady Show on the 4th December that the boys will be touring in summer 2010 and the dates are being announced soon. The tour is called "The Where We Are Tour". It was officially announced that they are the "2nd biggest selling artist" and "Biggest selling band of the decade" in UK with 10.14 million album sales.