Lion-O was born on Thundera to Claudus. He is the "Lord of the ThunderCats", a title that he inherited from Claudus. Lion-O He was only twelve years when he was forced to flee his home planet of Thundera, moments before its destruction.

During the flagship's long journey to Third Earth, Jaga instructs all the ThunderCats to enter into the Suspension Capsules to slow down their aging process. However, a malfunction occurred with Lion-O's Suspension Capsule which caused him to age faster then he should have. By the time they arrived at their destination, Lion-O had physically grown into an adult.

Lion-O carried with him the legendary Sword of Omens and the Claw Shield as his weapons. On Third Earth, he led the ThunderCats, helping them make new friends and allies, and fought against the mutants as well as Mumm-Ra.