A plague on Mars had claimed nearly all it's people. All but one. J'onn J'onnz (as he later became known), at the last minute was sucked through time and space into 1950s Earth. There, he instantly bonded mentally with his saviour, Dr. Erdel. To help J'onn forget the horrors of his life, Erdel implanted new memories in J'onn's mind,  memories of a Mars from pulp science fiction and fantasy. Along with these new memoires, J'onn developed a psycosematic weakness to fire. When the changes were completed, Erdel implanted memories of his own death, and let J'onn out, alone. 

Trapped on Earth, J'Onn used one of his natural talents to shape-shift his features so he could pass for a human. By altering his name to John Jones, and by brainwahing the precinct, he became a detective for the New York City Police Department. It wasn't until he moved to Denver Colorado, and met with Superman, who inspired J'Onn to use his powers publicly rather than in secret like he had been. 

Thus began J'Onn's career as the Martian Manhunter. Eventually he joined the Justice League of America, as a founding member, when aliens attempted an invasion. The Justice Leauge, though he's been absent a number of times, has been J'Onn's major significant work during his time on Earth. Since its inception, only he's been the one most dedicated to it, staying with it in all its incarnations, using his powers of shape-shifting, super strength, fasing abilities, telepathy, visionary powers, and flight, to lead the JL through many of its darker days. 

That was, at least people thought, until the JLA went through a breakdown and was reformed under Maxwell Lord. We at that time saw Superman lead the League, and a new member, Bloodwyn join the League. Bloodwynd was shrouded in mystery, until it was revealed to be J'Onn himself. He had no idea why he masqueraded as Bloodwynd, but on closer examination of the gem on his chest, it was discovered the real Bloodwynd was being held inside and tortured by his cruel beast that was the source of his power. Bloodwynd was freed, and J'Onn was free to go, but recontacted to join the League, as the leader of the Justice League Task Force, a group that had members selected from the rest of the heroes in the world based on the need of the mission. 

Then Judgement Day came, and the climatic events that took place broke up the League once more. J'Onn still stayed on as leader of the JLTF, but in a drasticly different form. The JLTF became a training group to train younger members of the JL for the future. However short-lived, lasting friendships were made. Now J'onn is once again a member of the JLA, along with the other great heroes of the time. He is rumored to have survived into the 30th century.