As a child, Maguire Beck idolized her older cousin Quentin Beck, a Hollywood special effects expert who became the super-villain Mysterio. When Quentin invited Maguire to join him in his life of crime, she readily accepted, using her computer expertise to help upgrade Quentin's special effects. In return, Quentin taught Maguire the secrets behind his illusions. Quentin also had two other students at the time: Daniel "Danny" Berkhart, Quentin's former cellmate who — during a time when Quentin had faked his own death — was hired by Daily Bugle publisher J. Jonah Jameson to pose as the ghost of Mysterio and capture Spider-Man, only to be arrested and then abandoned by Jameson; and Conundrum, an enigmatic student of Eastern mysticism who claimed to know nothing about his own past. Conundrum left to pursue his own ambitions and Berkhart adopted the costumed identity of Jack O'Lantern (previously used by Jason Macendale before becoming the Hobgoblin), a.k.a. Mad Jack, to pursue his personal vendetta against J. Jonah Jameson for abandoning him years ago. Though hired by Norman Osborn to target Jameson, Berkhart ultimately burned the cash to show his personal involvement. Maguire joined forces with Berkhart to hone her craft, usually remaining behind the scenes and using her computer technology to assist Berkhart, communicating with him via a radio transmitter built into a robotic cat. Maguire always thought that she and Quentin would eventually become full partners in crime — but then Quentin unexpectedly committed suicide. Devastated, Maguire dedicated her life to destroying everyone she regarded as having abused, exploited, or humiliated Quentin.
Upon learning of Quentin's suicide, Berkhart again became Mysterio and gave his Jack O'Lantern costume to Maguire, who became the new Mad Jack in order to distinguish herself from previous Jack O’Lanterns, preferring to orchestrate her illusions from behind a computer keyboard. As the new Mysterio, Berkhart briefly joined the Sinister Seven and then unsuccessfully attempted to determine Spider-Man's true identity, but later reaffiliated himself with Maguire (although Maguire's increasing mental instability led her to believe that the new Mysterio was actually Quentin). Together, the new Mysterio and Mad Jack plotted revenge against all those who had wronged Quentin. Mad Jack abducted J. Jonah Jameson. They kidnapped Quentin's ex-girlfriend Betsy Schneider, who had made a living selling toys and children's books using Mysterio's likeness, and her husband Joe Smith. Finally, the duo incapacitated Spider-Man and Daredevil (Matthew Murdock) by dousing them in hallucinogenic "Joy Juice" which negated their super-powers. Mad Jack and Mysterio placed their prisoners in virtual reality "Happy Tanks" where they were to be kept incapacitated forever; however, the heroes escaped and defeated Mad Jack, finally learning her true identity, although not realizing the involvement of Berkhart, who had escaped.