There were two Manhunters from Mars, J'onn Jon'zz who moved to Earth-One and Roh Kar who once visited Earth-Two. But they were not alone, as other heroes from these respective Mars were T'omm Jon'zz and Thund Dran, not to mention rogues such as B'enn B'urnzz and B'rett of Mars-One, as well as Sax Gola and Quork of Mars-Two. As a charter member of Earth-One's Justice League, J'onn had carved a niche for himself as a protector of the Denver metropolitan area of that era in the suburb of Middleton. Adopting the alter ego of police detective John Jones, the Manhunter used his various abilities to protect his adopted world. With superhuman strength, speed, vision, flight as well as shape-shifting and telepathy, J'onn was a formidable hero on par with Superman himself. After years of working his heroic feats while being invisible, when he was exposed in his true form to humanity he revealed himself to mankind as an alien protector in their midst. He would remain such until eventually returning to his then decimated homeworld, assisting in helping its survivors locate a new Mars to live on. After some time there, J'onn returned to Earth-One when he became a pariah to his people as a reminder to their failures as a society compared to his noble stand. At that point, J'onzz resumed his Justice League membership. Roh Kar was in pursuit of the criminal from his world known as Quork, who on Earth-Two had adopted the alias of the Stranger. To that end, Kar enlisted the aid of Batman and Robin who were operating in Gotham City where Quork had set up his criminal enterprises. Once the trio apprehended their alien adversary, the Martian Lawman return to his native world with his captive to continue as the golden age Manhunter of Mars. What the Dynamic Duo had failed to disclose with Roh during this case was of how some time prior to this adventure, they had been brought to Mars-Two by another Martian lawman known as Thund Dran, who sought their assistance in capturing the criminal mastermind of his world known as Sax Gola. Batman and Robin were given jet packs and weaponry in their fight against Sax Gola, the same tools which Roh Kar used when he visited their world years later.
Prior to leaving Eath-One, J'Onn assumed another identity after his John Jones alias was presumed deceased, that of international playboy Marco Xavier. Secretly a subcontractor for the villainous organization known as VULTURE, when Xavier seemingly perished his alter ego was assumed by the Manhunter, in order to bring it down along with its mysterious leader Mister Vulture. Turns out, Faceless was the original Xavier.
On Earth-Two, Marco's doppleganger was also wealthy playboy in Tex Thomson, who after a series of international adventures worked first for the local government in an American city as Mister America, then overseas in Axis controlled Europe as the covert double agent known as Americommando. A master of disguise, Tex was able to assume the identity of numerous German officers, while secretly undermining its influence with his alter ego. Using a specialized cape that enable him to fly, Mister America had a lengthy career on the side of justice... like the Martian Marco but unlike his actual counterpart in Mister Vulture/Marco... and was a member of the All-Star Squadron wherein he met counterparts to those members of the Justice League that J'onn befriended decades later.