The litany of public mass murders, from Aurora, Newtown, Charleston, Las Vegas, and Parkland to less well‐known incidents that occur yearly, has focused national attention on federally mandated mental health background checks of prospective gun purchasers. The call has been to put more gun‐disqualifying mental health records into the National Instant Criminal Background Check System database to prevent “deranged” murderers from buying guns and running amok. Our study examines whether increasing the robustness of the mental health background database will likely prevent potential public mass murderers from buying guns. Building on research that shows that serious mental illness contributes little to the risk of interpersonal violence and, further, that few persons with serious mental illness acquire gun‐disqualifying mental health records, we examine whether public mass murderers are among the small percentage of those with serious mental illness who do have gun‐disqualifying mental health records. Using a large sample of 106 US offenders who used a firearm to commit a public mass murder from 1990 to 2014, we find that half of the offenders had a history of mental illness or mental health treatment but that less than 5 percent had gun‐disqualifying mental health records. Implications of these findings and recommendations for further research are discussed.