Captain Marvel's arch enemy was the mad scientist Doctor Sivana, who regularly plotted elaborate schemes to rule the world and defeat both Captain Marvel and America. Sivana had four children who also had prominent roles in Captain Marvel's adventures. Two evil children, Georgia and Sivana, Jr., resembled their father both physically and in their desire to conquer the Earth. Sivana's daughter Beautia however, loved Captain Marvel and Sivana's son, Magnificus, was also a sometime-ally of the Captain.
Marvel's other adversaries included Captain Nazi and a nuclear-powered robot named Mister Atom. Matching Captain Marvel's adopted power from ancient heroes was Ibac, an evil magician who gained his power from the ancient villians Ivan the Terrible, Cesare Borgia, Attila the Hun, and Caligula.
One of the more popular serials in the Fawcett Comics run of Captain Marvel adventures was the Monster Society of Evil story arc which involved several of Marvel's adversaries from previous issues, including Doctor Sivana, Captain Nazi, and Ibac. The society was led by the cunning Mister Mind, a character who was eventually revealed to be a highly intelligent worm from another planet.