Lemnos was born at some point in the 30th century, somewhere in the United Planets. A psionic handicap he had would make his brain emit a quantum frequency that changes other individuals' chemicals and electrons related to memory. Hence, he could be in a crowd and be recorded by any electronic device, and after leaving, any memory or trace of him ever being there (or even existing for that matter) would disappear just within a few minutes. He claimed to Brainiac 5 the at he had to stay within his parents' line of sight so they wouldn't forget to feed him.
As it is next to impossible to remain completely undetected from everything in the 30th/31st centuries, the Science Police found enough traces of to track him down and capture him. They were desperate to find out how he had been able to stay hidden for so long, but unluckily enough for them, had already gained control over his ability. Such control that what used to be a handicap was now what could be called a superpower. With this, could easily manipulate the memories of anyone or anything, meaning he could trick an interrogator to believe he was alone in the interrogation room, for instance.
Rather than allowing himself to become some sort of lab rat for the Science Police, used his ability to spy on them, gathering as much valuable information as possible. Below their [the Science Police] noses, he was able to work behind the scenes to build his own technoempire. He figured that a man with his ability could anything without taking the responsibility or consequences for it...and hence become incredibly rich (stated by Brainiac 5 as being rumored to be the wealthiest individual in the known universe) and also shape every governmental decision to match his agenda.
Lemnos has one thing in common with the Legion of Super-Heroes - they both believe that the United Planets is a too slow and dull society for its own good. Which is why he supports the Legion in secret and has manipulated the outcome of numerous government decisions to help their cause. But for Lemnos, the peaceful "rebellion" of the Legion wasn't enough. He would take stronger actions to "help" the United Planets...even if it meant shocking them out of dormancy by declaring war. War was something could start by manipulating the outcast of the galaxy...the children of exiled criminals, leaved for dead by the UP. They became his army, and they were lead by some of the most powerful beings among them, called Terra Firma.
Lemnos's plan would have worked perfectly if it had not been for Dream Girl, who, during a precognitive vision, predicted that Lemnos (although his name was not known at the time) would have a connection to upcoming wars. Brainiac 5 was able to extract the image from her mind and upload it to his computer systems, although on a search, he couldn’t find any information whatsoever about him.
Praetor Lemnos’s first target in the United Planets was planet Orando, the economic centre. Lemnos’s armies were successful in destroying the planet and its inhabitants, leaving only Princess Projectra, who was on Earth at the time. Not long after, when Brainaic 5 was still trying to figure out who the mysterious warstarter was, Lemnos himself walked into Brainiac’s lab and introduced himself, and told about his origins and agenda. Including telling him that the next target was planet Colu, Brainiac’s homeworld. Brainiac refused to cooperate with him, leading Lemnos to walk out and erase Brainiac’s memories about their meeting. Brainiac though, had already sent a digital message to himself saying “COLU IS NEXT”, that he received a while later (leading him, Chameleon, Saturn Girl and Light Lass to go to Colu).
On planet Colu, Lemnos had already devastated the planet – but not the same way as Orando, by military force. On Colu, he had instead forced complete amnesia upon the population, rendering the scientific centre of the United Planet useless. Even though it was too late for the Legion to save Colu, Light Lass noticed that Chameleon’s antennae (which only appears when he is scanning a new lifeform) appeared for no apparent reason. She figured someone not visible was around them, and she destroyed the area by creating a massive anti-gravity field, forcing Lemnos to reveal his existence if he was to survive. escaped, but not before Brainiac 5 regained his memories and Saturn Girl seeing him (hence being able to shield every Legionnaire from memory manipulation).
Elysion, one of the members of Terra Firma, was captured by the Legion after a battle on planet Helegyn. Thanks to him, Saturn Girl could find out where Lemnos’s base world was, and hence the Legion set an investigation team to go there.
After a long battle (with reinforcements from the rest of the Legion), Lemnos’s armies along with Terra Firma were defeated. would have escaped if it hadn’t been for Brainiac 5, who opened a portal and stunned him with a seemingly powerful weapon. Lemnos is currently held in stasis by Brainiac 5.