According to the first chapter on Wildstorm’s comic Thundercats Origins: Heroes and Villains, Mumm-Ra’s origin is set in ancient Egypt. He was once Wahankh, a member of the pharaoh’s council and his most loyal adviser. We can also see Wahankh’s dog, Ma-Tep (supposedly the future Ma-Mutt). The friendship made with the pharaoh was so close and enduring that the sovereign actually considered him as part of his family.
However, Wahankh's real aim was to overthrow the royal power of his pharaoh in a coup d'état, and become the ruler of the land. To achieve his goal, he invoked the presence of the Ancient Spirits of Evil for the first time by using four magical runes, representing each one the four evil beings. Once the spirits appeared, he requested a small part of their power in order to defeat the powerful pharaoh’s army. In exchange, he would rule all Egypt in their name as a humble servant. The Ancient Spirits agreed to help him and Wahankh was invested with evil powers, becoming an immortal sorcerer at their service, while his entire body mutated into a cadaverous grey form. Finally, a new name was given to him: Mumm-Ra.
Knowing about this betrayal, the pharaoh immediately faced the monstrous and grotesque being. Trusting his new powers, Mumm-Ra ordered the pharaoh to bow down and proclaimed himself the new master of the realm. Nonetheless, the pharaoh’s son confronted him and quickly managed to outmanoeuver Mumm-Ra in battle, defeating him.
Without being aware that Mumm-Ra was actually immortal, the pharaoh spared him the fate of dying at the stake for his treachery. Considering all his years of service, the pharaoh instead granted him a merciful, religious death being locked as a mummy in an onyx pyramid’s giant burial chamber. The pharaoh's son, knowing about Mumm-Ra's neverending life, took joy in imprisoning him for (seemingly) all eternity. In the darkness, Mumm-Ra begged for the help of the Ancient Spirits once again, but they denied it due to his incompetence in using those powers they had previously granted him. Instead, they ordered him to build four large statues on the chamber to praise them, a task that would take at least 600 years to be accomplished. Only then –maybe- they would grant him the power to break free by turning into an eternal and overwhelming warrior, capable of defeating any pharaoh. Following the TV series storyline, at some point he fulfilled the work, receiving the ability to become Mumm-Ra, the Ever-Living.Wiki Source