Pablo Marcos was born in the small town of Laran, 180 kilometers from the Peruvian capital city of Lima. Marcos moved with his family to the capital at age five. His parents, Pablo (a taxi and gasoline-truck driver) and Maria Ortega Marcos, had four children at the time: Gloria, Berta, Pablo, and Manuel, later to be joined by Alfredo (who would become a cartoonist and caricaturist in Peru as an adult) and Oswaldo. While at the Bartolomé Herrera high school, Marcos studied under teacher and artist Juan Rivera Saavedra, who introduced him to the works of Argentine, Chilean, Italian and American comics artists such as Alberto Breccia, Arturo del Castillo, Hal Foster, Burne Hogarth, Hugo Pratt, Alex Raymond and Jose Luis Sallinas, among others.
Political cartoonist Julio Fairle had Marcos fill-in for him with spot illustrations in the influential Latin American newspaper La Prensa, which led to more newspaper work. Marcos later contributed caricatures to such weekly political magazines as Rochabus and Zamba Conuto while still an economics major at Peru's University of Lima. He married Norma Martinez in 1960, and the couple had a child, Judith, that same year.
During the 1960s, Marcos drew such comic strips as Benito Puna and James Bond 007 in Peruvian newspapers. He became art director of the newspaper Expreso, working as well on its evening edition, Extra, and a weekly supplement, Estampa. His and Martinez' second child, Gisella, was born in December 1963. Marcos became nationally known in 1965, following his illustrations for the trial and execution by firing squad of a convicted rapist. This wider recognition led to advertising artwork and high-profile political, news, and sports illustration. On December 29, 1966, his and Martinez' third daughter, Norma, was born, and Marcos began freelancing for the Mexican publishing company Editorial Novaro. The following year, while working on the Novaro series Legends of America, the Marcos family, including son Pablo, born December 19, 1967, moved to Mexico.
Marcos moved to New Jersey in the U.S. in the 1970s. Warren Publishing art director Billy Graham assigned him his first American-comics work, penciling and inking the six-page story "The Water World", by writer Buddy Sounders, in Warren's black-and-white horror-comics magazine Creepy #39 (May 1971). After another Creepy story and one in companion magazine Eerie that year, Marcos drew comics exclusively for rival Skywald Publications' Nightmare and Psycho from May 1972 to May 1973 cover-dates. Skywald co-founder Sol Brodsky introduced Marcos to fellow Peruvian artist Boris Vallejo, who became a mentor.
When Brodsky, who had been Marvel Comics' production manager, left Skywald to return to Marvel, he brought Marcos along as an artist and later his staff assistant for roughly two months. Marcos began drawing covers for such Marvel UK titles featuring such characters and features as Captain Britain, "Planet of the Apes", and Dracula. Marcos' naturalistic, "illustrative" style, similar to that of Neal Adams, became a mainstay of Marvel's black-and-white horror-comics magazines Dracula Lives!, Monsters Unleashed, Tales of the Zombie, Vampire Tales and others, and the exposure afforded by industry leader Marvel made Marcos a popular artist of the 1970s.
His first color-comics work in the U.S. was the cover of Marvel's Giant-Size Dracula #2 (Sept. 1974). Marcos' color-comics interior-art debut came at publisher Martin Goodman's short-lived Atlas/Seaboard Comics, illustrating the sword-and-sorcery title Iron Jaw #3 (May 1975). He went on to draw the following issue, plus the Iron Jaw story in Barbarians #1 and the cover of The Brute #3 (both July 1975) before the company folded.
Marcos next freelanced for DC Comics, drawing Man-Bat stories in Detective Comics, and working on an issue or two each of series including Freedom Fighters, Kamandi, Kobra, Secret Society of Super-Villains, and Teen Titans before returning to Marvel to do art for issues of The Avengers, The Mighty Thor and other comics. In 1980, Marcos additionally freelanced for an Italian comic-book series, Tremila Dollari per Ebenezer Cross Western Story, and created a series, "Dragon" for the Mexican magazine Ejea.