Objective. It is important to quantify the baseline mental health needs of the trauma population to improve the understanding of risk factors for poor patient outcomes . This will help guide improvements in care of vulnerable population groups. This review aims to describe the prevalence of pre-existing mental health disorders in trauma patients. Methods. This study was conducted according to the PRISMA guidelines. A total of 8,967 papers were identified through database searches. This was narrowed down to 43 for full text review following which 13 studies met criteria for inclusion. Results. There was a cumulative total of 1,710,367 patients, with 690,889 having a mental health disorder. This equates to a cumulative prevalence of 40.4% (with a range between 3.8% - 85%). Conclusion. There is a wide variation in prevalence of pre-existing Mental Health disorders in trauma patients. This may be due to methodological heterogeneity. Studies that interviewed patients or used patient self-reported questionnaires reported a higher prevalence than studies that relied on electronic medical records. Under-reporting of mental health disorders in electronic medical data bases may reduce the capacity to characterise the prevalence in the trauma population.