A dramatic coloratura soprano with a vocal range spanning over three octaves, Jansen began singing professionally with After Forever (then called Apocalypse) at age 16. She initially auditioned to be the backup singer, but the other members were so impressed they promoted her to lead vocalist, with guitarist Mark Jansen (no relation) providing death growls. She subsequently studied classical singing and musical theatre at the Dutch Rock Academy and the Conservatorium Tilburg. Following Mark Jansen's departure from the band in 2002, she became the primary songwriter.
In 2008, bandmate Sander Gommans began suffering from burnout following a performance at ProgPower USA. While there, Floor met up with Jørn Viggo Lofstad of Pagans Mind and performed a cover set of several classic hard rock songs. After Forever decided to take the following year off to wait for Gommans to recover, and over the summer Floor wrote most of a hard rock album with Lofstad, which after spending ten years in Development Hell finally came out in 2017 as Northward. After Forever ultimately broke up, and Floor formed a new band, ReVamp, with a significantly harsher and proggier sound.
After releasing ReVamp's Self-Titled Album, Floor herself began suffering from burnout. She took 2011 off to recover, and in 2012 she was at her sister Irene's wedding when she got the call that would catapult her to superstardom. Nightwish, whom she had previously met while touring with After Forever, had dismissed their then-vocalist Anette Olzon mid-tour and needed a replacement in a hurry. Floor flew out to meet them in California and had to learn the full setlist in the two days before the next show on the tour. The tour culminated in a headlining set at Wacken Open Air; her performance of "Ghost Love Score" from that show has become her Signature Song. Initially she was planned to only be a fill-in vocalist, but after the tour, the band asked her to stay on full time. The demands of her career with Nightwish unfortunately eventually led to ReVamp being disbanded.
In 2019 she participated in the Dutch singing competition TV show Beste Zangers. The single version of her performance of "The Phantom of the Opera" with Henk Poort (the originator of the role of Eric in the Netherlands) reached #1 on the Dutch music charts.
In 2021, she announced she was working on a solo album. Her first-ever solo single "Fire" was released on 25 March 2022. On September of the same year she also performed the Dutch national anthem at the Formula One Dutch Grand Prix.
She currently lives outside Gothenburg, Sweden with her husband Hannes Van Dahl (drummer for Sabaton), their daughter Freyja, and several horses, cats, and dogs. Her younger sister Irene Jansen also sings and has performed with her on several occasions. She's also old friends with former Nightwish vocalist Tarja Turunen and with several members of Epica, which former bandmate Mark Jansen and his then-girlfriend Simone Simons formed after he left After Forever.
- Prison of Desire (2000)
- Decipher (2001)
- Exordium (EP, 2003)
- Invisible Circles (2004)
- Remagine (2005)
- After Forever (2007)
- ReVamp (2010)
- Wild Card (2013)
- Endless Forms Most Beautiful (2015)
- Human. :II: Nature (2020)
- The Last Stand (choir, 2017)
- The Soundtrack to The Great War (2019)
- The Symphony to End All Wars (2022)
- Universal Migrator Part 1: The Dream Sequencer (2000)
- 01011001 (2008)
- The Source (2017)
- Timo Tolkkis Avalon:
- Angels of the Apocalypse (2014)
- Quarterpast (2011)
- Antagonise (2014)
- Star One:
- Space Metal (2002)
- Victims of the Modern Age (2010)
- Revel in Time (2021)
- A Paranormal Evening with the Moonflower Society (2022)
- Northward (2018, with Jørn Viggo Lofstad)
- untitled solo album (2023)
- Costume Porn: Floor's stage outfits with Nightwish have gotten progressively more extravagant over the years.
- Genre Roulette: While she's best known as a Symphonic Metal singer, she also loves Hard Rock and is trained in classical and musical theater performance. In her appearances on Beste Zangers she sang covers of everything from "The Phantom of the Opera" to "Shallow" by Lady Gaga. "Fire" is an Indie Pop number with orchestral backing.
- Harsh Vocals: She learned to perform Death Metal growls for ReVamp's second album, though she rarely breaks them out these days.
- One-Steve Limit: Floor and Mark Jansen have been known to joke about people mistakenly thinking they're siblings.
- One-Woman Wail: The "Floorgasm", which is particularly used to refer to the extended vocalization that closes off her perfomances of "Ghost Love Score".
- Statuesque Stunner: At 183 cm (6') barefoot, Floor is taller than most of her male bandmates, and she's been known to use it to intimidate hecklers. It led to fandom jokes when Wintersun bassist Jukka Koskinen was announced as a temporary replacement for Marko Hietala in 2021 that Nightwish had finally found someone taller than her.
- Truck Driver's Gear Change: Floor is famed for her vocal agility, able to switch octaves and singing styles at the drop of a hat.