Background:: Using three patient characteristics, including age, Injury Severity Score (ISS) and transfusion within 24 h of admission (yes vs. no), the Geriatric Trauma Outcome Score (GTOS) and Geriatric Trauma Outcome Score II (GTOS II) have been developed to predict mortality and unfavourable discharge (to a nursing home or hospice facility), of those who were ≥65 years old, respectively. Objectives:: This study aimed to validate the GTOS and GTOS II models. For the nested-cohort requiring intensive care, we compared the GTOS scores with two ICU prognostic scores – the Acute Physiology and Chronic Health Evaluation (APACHE) III and Australian and New Zealand Risk of Death (ANZROD). Methods:: All elderly trauma patients admitted to the State Trauma Unit between 2009 and 2019 were included. The discrimination ability and calibration of the GTOS and GTOS II scores were assessed by the area under the receiver-operating-characteristic (AUROC) curve and a calibration plot, respectively. Results:: Of the 57,473 trauma admissions during the study period, 15,034 (26.2%) were ≥65 years-old. The median age and ISS of the cohort were 80 (interquartile range [IQR] 72–87) and 6 (IQR 2–9), respectively; and the average observed mortality was 4.3%. The ability of the GTOS to predict mortality was good (AUROC 0.838, 95% confidence interval [CI] 0.821–0.855), and better than either age (AUROC 0.603, 95%CI 0.581–0.624) or ISS (AUROC 0.799, 95%CI 0.779–0.819) alone. The GTOS II's ability to predict unfavourable discharge was satisfactory (AUROC 0.707, 95%CI 0.696–0.719) but no better than age alone. Both GTOS and GTOS II scores over-estimated risks of the adverse outcome when the predicted risks were high. The GTOS score (AUROC 0.683, 95%CI 0.591–0.775) was also inferior to the APACHE III (AUROC 0.783, 95%CI 0.699–0.867) or ANZROD (AUROC 0.788, 95%CI 0.705–0.870) in predicting mortality for those requiring intensive care. Conclusions:: The GTOS scores had a good ability to discriminate between survivors and non-survivors in the elderly trauma patients, but GTOS II scores were no better than age alone in predicting unfavourable discharge. Both GTOS and GTOS II scores were not well-calibrated when the predicted risks of adverse outcome were high.