Abstract:First responders and veterans are two populations that have been identified to be at heightened risk for experiencing suicidality. Nationally, veterans comprise between 10 to 25% of the first responder population. However, little information exists about the overlap between these two populations and how their professional work experiences influence suicidality. Using data collected through the Symptom Assessment for Emergency Responders (SAFER), the purpose of this dissertation was to identify differences in suicidality and in mental health factors that impact suicidality among first responders with and without prior military experience. Results indicated that first responder veterans reported higher levels of both suicidality and substance use compared to their non-veteran counterparts. Furthermore, suicidality was positively correlated with several clinical symptoms, including substance abuse, arousal/reactivity, intrusive symptoms, avoidance, negative alterations in cognition and mood, and depression. These findings suggest that increases in these psychological symptoms are generally associated with heightened suicide risk among first responders. Notably, no significant differences were observed between first responder veterans (FRVs) and non-veterans in the endorsement of posttraumatic stress disorder (PTSD) symptoms or depression, indicating that elevated suicidality among FRVs may be more closely tied to other factors, such as substance use or accumulated occupational stress. These results underscore the critical need for targeted interventions that address the complex interplay between mental health symptoms and suicidality, particularly among first responders with military service backgrounds.