Born and raised in Madrid and currently living in London, Guillermo first started inking comic books professionally after studying History at university. Also working as a freelance illustrator on the side, he rapidly moved into the American scene breaking into Marvel Comics on titles such as Wolverine Vs. Thor and X-Factor which was written by Peter David and drawn by artist Emanuela Lupacchino. He has also collaborated with companies like DC comics, Valiant and Dynamite. Some of the titles he has worked on are Green Arrow, JLA, Justice League Dark, Nightwing, Archer & Armstrong, Bloodshot, the H.A.R.D. Corps and more.