Juanjo Guarnido got his degree in fine arts in birthcity Grenade. He cooperated on several local fanzines, while also made illustrations for the Spanish Marvel publications (Comics Forum - Planeta de Agostini). He worked in an animation studio in Madrid for three years, where he met his future scriptwriter Juan Diaz Canales. Guarnido moved to Paris in 1993 and was employed by Walt Disney Studios in Montrieuil as a lay-out man and animator on films like 'Tarzan', 'Hunchback of the Notre Dame' and 'Hercules'.
In his spare time, he started making comics together with Canales. This eventually resulted in the launch of their critically acclaimed anthropomorphic black cat detective 'Blacksad', of which the first book was published by Dargaud in 2000. Guarnido's film noir-inspired comic is inhabited by a great many remarkable and unusual animal characters, including anthropomorphic cameos of famous persons and a wide range of reptile villains. The series is published in French, Spanish, Bulgarian, Catalan, Chinese, Croatian, Danish, Dutch, Czech, English, Finnish, German, Greek, Italian, Japanese, Norwegian, Polish, Portuguese, Russian, Serbian, Slovak, Swedish, and Turkish.
Guarnido was also the cover artist and illustrator of the final installment for Pierre Boisserie's collective science-fiction series 'Voyageur', that was published by Glénat between 2007 and 2011. With Canales' wife Teresa Valero he additionally set up the heroic fantasy children's series 'Sorcellières', that is published by Dargaud since 2008.