Mumm-Ra is the chief villain and antagonist against the ThunderCats. He is the self-proclaimed "ever-living source of evil" on Third Earth, having powers of sorcery and an apparently unlimited lifespan. He is, in fact, a bound servant to the Ancient Spirits of Evil who provide him with increased power and virtual immortality to further his pursuit of spreading their dark influence throughout Third Earth. He was voiced by Earl Hammond on the original 1984-89 Thundercats, and by Robin Atkin Downes for the 2011 reboot.

Residing within the Black Pyramid amid the ruins of what appears to be an ancient Egyptian civilization, Mumm-Ra exists in a decayed, weakened form that must return to a stone sarcophagus to replenish his energy. When leaving the pyramid, he can summon the power to transform himself into a more vigorous form called the Ever-Living form, by reciting an incantation. While in this form, Mumm-Ra possesses superhuman strength, the ability to fly, as well as fortification of his mystical might, such as casting spells, throwing energy bolts, etc., to battle his foes. He can also alter his physical form into a variety of alter-egos to deceive his enemies.

Ever watchful, Mumm-ra, in his Ever-Living form, watches Brian Griffin while he poops in "The Man with Two Brians". On his show however, he would observe beings in his decayed form.