Panthro started out as a young soldier in the Thunderian army where he would meet and become friends with Grune and Claudus. He and Grune rose quickly through the ranks, eventually becoming generals in the service of Claudus. They also served as teachers to Lion-O and Tygra.
Claudus sent Panthro and Grune to search for the fabled Book of Omens. The two roamed the furthest corners of Third Earth but instead of the book, they found the evil Mumm-Ra. Grune joined forces with Mumm-Ra, betraying Panthro and throwing him down an abyss.
Panthro however survived the fall and after enduring a grueling journey through the desert and escaping from the enslavement in the City of Dogs, Panthro returns driving the ThunderTank and meets up with Lion-O and the other fleeing ThunderCats.