He graduated from the Helsinki Pop & Jazz Conservatory at the age of sixteen and was subsequently invited by Jaska Raatikainen to join Children of Bodom, due to their previous keyboardist, Jani Pirisjoki, being fired from the band.

This synchronization can be observed during their live performances, or on the recorded tracks Bed of Razors, Lake Bodom and Downfall.

In the same year, Johansson was asked by heavy metal band Masterplan to play on their debut album, but since he was already busy with other projects, he referred to Janne instead.

The band proceeded to offer him a permanent position, but Warman opted to continue playing with Children of Bodom.

On the Chaos Ridden Years documentary, Laiho said, "But it was the first time I saw the guy drunk, and that's when I knew he was the fucking man."