Recent high-profile cases and scientific research on lone actor terrorist attackers have indicated that there may be an association with mental illness. Simultaneously, countering violent extremism (CVE) initiatives have recently been placing more emphasis on building intervention (aka secondary prevention) programmes to address this matter, with mental health professionals possibly playing important roles in preventing lone actor terrorist attacks. Mental health professionals can contribute to intervention programmes, as practitioners and as leaders, but more so by drawing upon the threat assessment model, than because of a possible association with mental illness. A public-private partnership in Los Angeles, based on a well-regarded, community-based service for addressing targeted school violence, is attempting to further develop such services. A tabletop exercise was used as a strategy for jumpstarting interventions, so as to engage mental health and other community partners, build trust between stakeholders, and identify capacities and gaps that need to be addressed to ensure successful implementation. Further progress and future success will depend upon equitable, ethical, evidence-based, and community collaborative practices.