Vallejo commenced painting at the age of thirteen and had his first illustration job at the age of sixteen. After emigrating to the United States in 1964, he quickly garnered a fan following from his illustrations of Tarzan, Conan the Barbarian, Doc Savage and various other fantasy characters (often done for paperback fiction works featuring the characters).
This led to commissions for movie poster illustration, advertisement illustration, and artwork for various collectibles - including Franklin Mint paraphernalia, trading cards and sculpture. Along with Bell, Vallejo presents his artwork in an annual calendar and various books. Vallejo's work is often compared to the work of Frank Frazetta, not only because it is similar stylistically but also since Frazetta painted covers for paperbacks of some of the same characters.