Following the collapse of the Soviet Union, he traveled to America, where he briefly worked at the FBI. He worked on several cases as a psychometrist, requiring him to dive into the psyche of a serial killer, warping his own mind into that of the killer's in the process. After committing murder himself, Mantis left the FBI to pursue freelance intelligence work until his eventual scouting by FOXHOUND.
During the Shadow Moses Incident, he participated in Liquid Snake's revolt as a member of FOXHOUND, using his powers to brainwash the Genome Soldiers. He attempted to use his mind-reading powers to extract the nuclear launch codes from the DARPA Chief and ArmsTech president, but was foiled by the psychic insulation surgery they had previously undergone.
Ultimately, he fought against Solid Snake using his psychokinetic abilities. During their battle, Mantis peered into Snake's mind and found the same emptiness he himself held, but was defeated in the end. The hour of his death also marked the first time he ever used his powers to help someone. His psychic powers occasionally spawn afterimages that persist within surrounding objects. Even after death, his psyche lingers on in the grounds of Shadow Moses.
Psycho Mantis' residual psychic energy can drive other people to madness. It was that energy that served to emotionally and physically control the Beauties in the BB Corps. Psycho Mantis' thoughts found a host in one of the Beauties, who become Screaming Mantis.