Jason is the son of Emperor Eson of Spartoi, and Prince and heir to the royal throne. He is very idealistic, which causes the ruling Council of Ministers to prevent him from taking his father's place as ruler. As the rules of education of his culture dictate, he grew up on dozens of planets with dozens of professions including miner, poet, servant, soldier and pilot, the latter which he seems to prefer. He is also tutored in many different fields.
When Ronan the Accuser of the Kree Empire forced the Inhuman Royal Family to attempt to assassinate the Majestrix of the Shi'ar Empire Lilandra during a wedding ceremony that would symbolically unite the Shi'ar and Spartoi, Jason was accused of being part of the plot since he had been seen with the Inhumans. He was banished and deemed unworthy of his title.
He would be summoned to return home by his father when war broke out between Spartoi and the Ariguans. On his way, he was forced to crash land on Earth, where he fell in love with a woman named Meredith Quill. He would be forced to leave and continue his efforts in the war, but not before locking her memories to shield her from the pain of his departure. She would later give birth to his son, Peter Jason Quill.
Jason of Spartax first appeared in the Star-Lord story by Chris Claremont in Marvel Preview #11 (6/77). These original appearances would later be moved from the mainstream Marvel Universe and dubbed Earth-791.
J'Son of Spartax first appeared in Inhumans #2 (7/00). While originally heroic, he would become a crooked and disgraced ruler and master criminal.
Jason would eventually go on take his father's place as Emperor of Spartoi. He would meet his son after he became Starlord, offering him a place as emperor. Peter refused and in his stead Jason adopted Kip Hölm as future heir. As emperor, Jason would rule for 100 years before leaving the Empire to Kip to finally form a relationship with his son.
In Other Media
J'son appears in the Guardians animated series, voiced by Star Trek alum Jonathan Frakes. As a young man, he stole a powerful artifact called the Cosmic Seed from Asgard, leading to hostilities between the two worlds. He also ordered his son Peter abducted by Yondu in order to mold him into a worthy heir. It is later revealed that J'son had partnered with Thanos to destroy the Asgardians in exchange for the Seed.