Thornton's early career and lyrics with the Ultramagnetic MCs often referenced his time as a patient of Bellevue, a psychiatric hospital, where he was treated for depression. With Ultramagnetic, his abstract style developed, maturing in songs like "Poppa Large." He was probably the first rapper to create an alter ego, and then create an alter ego for that alter ego (Kool Keith v. Moe Luv). After Ultramagnetic broke up, he expanded his resume, experimenting with new styles, including his self-described "pornocore," known for pornographic imagery and brutal, abstract lyrics. Sex Style released in 1997, was the showcase for this genre.
Teaming up with Dan "the Automator" Nakamura under the name Dr. Octagon, Thornton again reinvented his image, recording the self-titled Dr. Octagon album (later reissued under the title Dr. Octagonecologyst) which was released by the English trip hop label Mo' Wax. The album received critical acclaim, though mainly from sources outside the mainstream hip-hop community. The critical success of the album helped to open the accessibility of the underground hip hop scene to an appreciative audience. For the Automator, this album laid the foundation for a burgeoning career and accolades as a producer; for Thornton, the success of Octagonecologyst seemed to put him in danger of being typecast as a "space and Mars" rapper, or an industry tool, which he throughout his career has looked down upon.
His style as a whole is characterized by non sequiters and bizarre commentary, particularly anal, fecal and genital references. For example, "Chief administrator suck my oil," "Dr. Ludicrous I turn into an octopus," "All this jealousy shit is mayonnaise and celery shit," "Two drinks minimum, don't worry, we're maximum," etc. An instrumental version of the album, Instrumentalyst, showcased the production, became a rising success with most of the credit given to the Automator and other collaborators, even though Thornton is credited on the LP with some production duties.
Frustrated with the limits of his new image, Thornton abandoned the "Octagon" character, who was literally killed off in the introduction to his next album, Dr. Dooom. Perhaps as a jab at Instrumentalyst's success, he has released instrumental versions of many post-Octagon albums.
While Octagonecologyst was many fans' first taste of his style, still others discovered him on The Prodigy's 1997 album The Fat of the Land. Long time fans, the Prodigy invited Kool Keith to supply the lyrics on "Diesel Power" after they used a sample from Ultramagnetic that became the lyrics and title for their (soon-to-be) hit "Smack My Bitch Up." Thornton sued Prodigy for failing to clear the sample, claiming he no longer used the misogynist language and imagery he did at that time. Previously, Prodigy frontman Liam Howlett had sampled Thornton's lyric "Pay close attention, I'll take your brain to another dimension" (on the single "Out Of Space") without incident.
Released simultaneously with the independent Dr. Dooom was major-label effort of Black Elvis/Lost in Space. While Dr. Dooom was produced with the minimalist beats and production work of Kutmasta Kurt, Black Elvis featured much more complex lyrical styling and deeper, multi-layered tracks developed by a host of collaborators. The dichotomy lead to something of a division between Keith's long-time fans: many regard the minimalist work with Kutmaster Kurt (e.g. Sex Style, Dr. Dooom, and the later Matthew) to be the authentic Kool Keith, while others, longing for a return to the deeper and more complex work on Octagonecologyst and Black Elvis, were disappointed with what they considered to be mediocre, or even shoddy work.
In 2000, Thornton released Matthew, which included the widely-cited anti-label rant "Test Press", lambasting mainstream rap record labels as being "white" and damning them for manipulating groups, artistic decisions, and profits. Subsequent collaborations have included Masters of Illusion, Clayborne Family, the Analog Brothers and his current effort (as of 2004) with Thee Undatakerz, who have collaborated with members of the Wu Tang Clan to produce the soundtrack to Blade: Trinity. Thornton also appeared with ODB on the compilation album WWF Aggression, on a song featured as a tribute to World Wrestling Entertainment (WWE) superstar Mankind. He is also a well-known favorite of Princess Superstar, and has collaborated with her on several tracks.
In early 2006, Keith contributed to a track for former Faith No More vocalist Mike Patton's "Peeping Tom" album (released June 30, 2006).
Thornton played Crazy Lou, a gun dealer and former Marine who was, "dishonorably discharged for sexual misconduct with a deadly weapon," in Prince Paul's "hip-hopera" A Prince Amongst Thieves.
Kool Keith's next single is expected to be named "Break U Off" with Kutmasta Kurt