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. Floor's 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.[1] Prior to this, up until the Decipher (2001) album, the two would write together.
Floor 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 on, she started teaching performing in her own course called Wanna be a Star?!.
Floor plays the guitar, piano and flute and has completed several years of music studies, and has a Dramatic Soprano vocal range[citation needed].
Three albums by the Dutch composer and multi-instrumentalist Arjen Anthony Lucassen include guest vocals from Floor. She sang one song on Universal Migrator Part 1: The Dream Sequencer, made a number of contributions to the Star One project and plays the character ? in the Ayreon album 01011001. She is the older sister of fellow singer and Ayreon guest vocalist Irene Jansen, who played "Passion" on The Human Equation. She has also been a guest for the metal band Nightmare.