He appeared as the main antagonist in the Season 8 episode "Toxic".
He was portrayed by Ron Selmour, who also played Connelly in the same series.
Marcos and his gang came upon a group of marine biologists and killed several of them but left Tess Mercer and her friend Megan alive. They took them to an island where they found Oliver Queen who had been poisoned by a flowering plant.
Tess nursed him back to health with leeches so he could be ransomed. Marcos entered their tent where he approaches Oliver and tells him that he knows his identity.
He knows that Oliver is a rich heir and announces plans to ransom Oliver. He then shot Megan dead and almost shoots Tess too, but Oliver jumped in front of her. Marcos decided not to kill Tess.
Marcos and his gang prepared to leave the island on a boat. Oliver and Tess get ready to board Marcos' boat, but Oliver had fashioned a dart. He shot Marcos with an arrow covered in poison of a flower that grew on the island, wounding and poisoning Marcos.
Oliver told him about using the leeches, if he could find them. The pair then left on the boat leaving a poisoned Marcos behind.
Marcos survived his encounter and came to Metropolis for revenge. First he found Oliver at a fundraiser and poisoned him and his date Adrianna with the same poisoned flower from the island. Adrianna died from the poison when she was given adrenaline but Oliver was saved just in time when Clark Kent cured him with an antidote from LuthorCorp.
Oliver awoke telling Clark that whoever poisoned him will target Tess next and that Clark go and save her.
Marcos then left for the Daily Planet, slashing Tess Mercer's tires so she couldn't escape. Marcos attacks her from behind and, although she fights back, he eventually punches her out. Just as he is standing over her ready to kill her, Clark arrives and uses his heat vision to make a power line fall and knock Marcos out.
Marcos was arrested by the Metropolis police, but Tess paid his bail. As he prepared to leave town in a cab, Tess approaches him and scratches his hand with the same toxic flower.
As the poison began to take effect, Marcos passed out and Tess told the taxi driver to take him to a hospital, knowing that they would likely treat him with adrenaline. The adrenaline would only hasten the effect of the poison, killing Marcos much faster.