Hyde is well known for providing vocals in every song by Underworld. He founded the group along with Rick Smith. Karl had a pop band, the Screen Gemz and badgered Rick Smith into joining whilst at Cardiff art college.

In 1983 they recorded two albums for CBS Records International with a proto-electroclash, new romantic band, whose name was a graphic squiggle, which was soon given a pronunciation, Freur.

Freur disbanded in 1986. In 1987 members of Freur created the band Underworld and tried a more guitar-orientated funky electropop sound on two albums for Sire Records before disbanding in 1990. (The Underworld of that period is now often referred to as "Underworld Mk1").

Hyde and Smith are the longest serving members of the musical project and are still together performing today. He is a self confessed alcoholic[citation needed], and has been the writer for many of their songs.

He states that the lyrics in the song Born Slippy .NUXX were like that of a drunk, as he was when he was writing the high selling single.