Firefighters (FFs) protect the public despite significant risks to their health and well‐being stemming from frequent trauma exposure and other occupational stressors. A minority of FFs develop posttraumatic stress disorder (PTSD) or related mental health problems, whereas most remain remarkably resilient despite enormous stress. This points toward substantial variability in responses to traumatic stress among FFs. Personality, particularly negative emotionality (NEM), has been shown to predict the development of PTSD in other trauma‐exposed populations, yet has not been prospectively studied in relation to PTSD in FFs. The aim of this secondary analysis from a broader study of mental health in FFs was to test whether preemployment NEM predicted PTSD symptom severity over time by influencing how FFs respond to traumatic experiences. In this first prospective study of the development of PTSD symptoms in professional FFs, 322 FFs were recruited from seven urban fire academies across the United States and followed over their first 3 years of fire service. We assessed NEM during the fire academy as well as trauma exposure and both self‐reported and clinician‐rated PTSD symptoms at 1‐, 2‐, and 3‐year follow‐ups. Level of trauma exposure and NEM predicted PTSD symptoms over time, and NEM moderated the effect of trauma exposure on clinician‐rated PTSD symptoms across both trauma exposure measures at 1‐ and 3‐year follow‐ups, f
2 = .03–.10, but not at 2‐year follow‐up nor for self‐reported PTSD symptoms. These findings indicate that NEM, assessed upon entry into a high‐risk occupation, is useful in predicting PTSD symptom development.