Jansen had been the lead vocalist of symphonic metal band After Forever that she joined in 1997. After they disbanded in 2009, she created ReVamp and released two albums with the band. In 2012 following the departure of their lead vocalist Anette Olzon, Nightwish replaced her with Jansen as a session member until the end of their Imaginaerum World Tour. In 2013 the band announced her as their now full-time lead vocalist.
Jansen was sixteen, when she joined Apocalypse (the name of the band After Forever in its early days) in 1997. Three years later the band released its first album, Prison of Desire. Her ability to sing both classical and rock music have made her reasonably popular in the metal scene. She took sole charge of After Forever's lyrics and vocal melodies since the departure of Mark Jansen (no relation) in 2002. Prior to this, up until the Decipher (2001) album, the two would write together.
Jansen started studying music at the Dutch Rock Academy in 1999 and entered the conservatory three years later. She studied musical theatre and one year of opera. Later, she started teaching performing in her own course called Wanna be a Star?!.
Jansen plays the guitar, piano and flute and has completed several years of music studies, and has a sopranovocal range.
Because of the burnout that After Forever bandmate Sander Gommans underwent, the band took a year off starting in January 2008 but ultimately decided to call it quits in February 2009. Jansen posted on her website, that while After Forever was on hold (and since then disbanded), she would use this opportunity to start writing music with Jørn Viggo Lofstad (no) (Pagan's Mind, Jørn Lande) for a new musical project. On 16 June 2009, Jansen announced through her MySpace site, that she had started a new metal band, which has put her project with Lofstad from Pagan's Mind (called Sinh) on hold.
In August 2011, Van Canto announced on Facebook, that ReVamp would no longer participate in their Out of the Dark tour, as Jansen suffered a burnout herself.
In 2011, Jansen joined MaYan's Latin American tour, performing in São Paulo. In 2012, Jansen joined Adrenaline Mob in Bochum and Weert. Currently ReVamp is working on their second album, which is scheduled to be released in August 2013. Revamp's album will be called Wild Card, a title that, according to singer Jansen, came about as follows: "This album became the most aggressive album I've ever made and in the lyrics I tell a lot about my personal experiences of the last years. This resulted into some heavy stories and some openhearted 'screams' I needed out of my system. It matches the violence of the music in a great way, we wrote as a great team! But not one sentence or word can really cover the contents of this album. Its variety in all its ingredients makes it almost impossible. A wild card is an unpredictable and unforeseeable factor. You don’t know what you’re getting, or when. That's this album! It's 100% ReVamp and it's your wild card!"
Jansen acted as Nightwish's live lead vocalist for the remainder of the Imaginaerum World Tour, after Nightwish's previous lead vocalist Anette Olzon parted ways with the band. On 9 October 2013, Jansen was announced as Olzon's permanent replacement in the band.
On December 9, 2013, she was also announced as the main vocalist, playing the lead role, for the second full length album of former Stratovarius guitarist Timo Tolkki's metal opera act Avalon.