Objective Although it is well established that neuroticism increases the risk of posttraumatic stress disorder (PTSD), little is known about the mechanisms that promote PTSD in individuals with elevated levels of neuroticism. Across two studies, we examined the cognitive‐affective processes through which neuroticism leads to greater PTSD symptom severity. Method Community‐dwelling adults with trauma histories varying widely in severity (Study 1) and clinically diagnosed individuals exposed to DSM‐IV‐TR A1 criterion traumas (Study 2) completed measures of neuroticism, negative affectivity, trauma memory characteristics, and PTSD symptom severity. Results Longitudinal data in Study 1 showed that individuals with higher scores on two measures of neuroticism assessed approximately three decades apart in young adulthood and midlife reported trauma memories accompanied by more intense physiological reactions, more frequent involuntary rehearsal, and greater perceived centrality to identity in older adulthood. These properties of trauma memories were in turn associated with more severe PTSD symptoms. Study 2 replicated these findings using cross‐sectional data from individuals with severe trauma histories and three additional measures of neuroticism. Conclusions Results suggest that neuroticism leads to PTSD symptoms by magnifying the emotionality, availability, and centrality of trauma memories as proposed in mnemonic models of PTSD.