Previous research highlights inconsistent associations between premorbid hair cortisol concentrations (HCC) and posttraumatic stress disorder (PTSD) symptoms, often neglecting the critical role of childhood trauma (CT) in civilian populations. To address this gap, our study investigates the predictive value of HCC for PTSD symptoms following a motor vehicle crash (MVC), extending our prior findings by assessing CT as a moderator within a sample that includes participants with and without CT. We hypothesize that pre-MVC HCC is positively associated with PTSD risk and that this relationship is moderated by early adversity. We examined N = 272 participants with a traumatic brain injury aged 18–65 years who experienced a MVC between 2010 and 2020. Cortisol concentrations were determined in 3 cm scalp-near segments of hair samples that were obtained at the emergency room shortly after the MVC (t1). Participants completed measuring instruments capturing symptoms of posttraumatic stress (Posttraumatic Diagnostic Scale [PDS]; Impact of Event Scale-Revised [IES-R]) and Childhood Trauma Questionnaire (CTQ). PDS and IES-R were re-collected three months post-MVC (t2). Elevated pre-MVC HCC predicted PTSD symptoms (p < 0.05), emphasizing the role of chronic stress and HPA axis dysregulation in PTSD. Contrary to our hypothesis, CT did not moderate this relationship, suggesting that HCC’s impact on PTSD is independent of early adverse experiences. In this context, CT emerged as an independent predictor of PTSD at the 3-month follow-up, underscoring its lasting influence on psychological trauma vulnerability, particular in the face of recent adversity. Our study confirmed that elevated pre-MVC HCC levels predict PTSD symptoms. Although childhood trauma did not moderate this relationship, it independently predicted PTSD at follow-up. These findings underscore the lasting impact of early adversity on mental health, highlighting the importance of considering both HPA axis regulation and trauma history to develop targeted interventions for adults exposed to new stressors.