Saiz’s career began in Spain. He trained as an oil painter at an art academy, and had planned on following that career path. As a kid, he’d always enjoyed comic books and liked drawing, though it wasn’t until he met his friend David Macho that he realized that he could actually have a career as a comic book artist. With Macho’s help, he worked on small collaborative projects at smaller publishers until coming the US, where he began working on the Dark Horse titles Iron Reich 3000 and Star Wars Tales.
Although he has done work on several superhero titles for DC, including Birds of Prey in the New-52 and Resurrection Man, Saiz’s primary influences are older Conan titles, which have had a major influence on his "Beowulf" story.