A heroic scientist, Dr. Mnemonic was instrumental in defeating Nemesis during that villain's 1945 attempt to conquer America.
When the villain known as Nemesis "conquered" America in 1945, part of his plot involved hiding nerve gas bombs throughout the twenty largest cities in America. With the touch of a button he released the deadly toxin, ensuring a painful death to tens of millions if they did not receive the antidote within 24 hours. Sister Psyche used her telepathic powers to locate the antidote. Soon after, Dr. Mnemonic synthesized an airborne version of the remedy, which was spread over the infected cities by the air force and numerous flying heroes. With Nemesis' nerve toxin neutralized, the world's heroes handily defeated the villain's forces. Nemesis, however, escaped.
Although clearly active in 1945, it is unknown whether Dr. Mnemonic continued to serve as a superhero, nor whether he survived such conflicts as the Rikti invasion. It is assumed that he is dead or inactive.