Samuel Toth and Kyle Nimbus were scientists who tapped into scientific principles which caused their own molecular structure to destabilize. Becoming gaseous entities evading detection and enabling them to become malleable menaces, Toth became Silver Fog and Nimbus became the Mist.
On Earth-Two, the Mist twice faced the superhero known as Starman, who each time found a way to defeat the invisible criminal. Years later, coming out of retirement, the Mist employed fellow felons to become his invisible legion, although these also faced defeat at the hands of Starman and his teammate on the Justice Society, Black Canary. It would be decades before he returned.
On Earth-One, Silver Fog had a less ignoble reason for his criminal career, attempting to find the tools to reverse his process caused by his particle accelerator. First facing Doomster and the Grasshopper, the temporary heroic aliases of teenagers Christopher King and Victoria Grant, Toth retreated to plan anew, now realizing he would have obstacles in his path to his goals.
Shortly thereafter, Fog faced the pair who power dialed into the heroes known as Captain Electron and Windsong. It was Electron whose charged blast helped Silver Fog to control his being. Having found a cure, Toth turned himself over to the authorities to pay for his crimes.
Meanwhile, the Mist journeyed to Earth-One to face Black Canary in a rematch on board the Justice League satellite, in order to capture her for a plot by his criminal cohorts the Secret Society of Super-Villains. When this scheme ultimately failed, Mist eventually returned to Earth-Two where he traveled back to the 1940's with the Society to battle the All-Star Squadron, and once more be vanquished.