Introduction Major Trauma Hospitals are receiving increasing numbers of older patients after serious injury. Outcomes in these patients vary with the nature of the injury and other factors such as frailty. We aimed to determine the prevalence of frailty and adverse events in older patients managed by acute trauma services during the index hospital admission, and the frequency of adverse outcomes at three, six and twelve months after discharge in an Australian setting. Methodology This study assessed the prevalence of frailty in a prospective multicentre cohort study of seriously injured patients aged ≥ 70 years admitted to three Major Trauma Services in Australia. Patients were followed for twelve months after injury to assess for adverse outcomes associated with the presence of frailty or other factors. During the index admission patients were assessed for frailty, co-morbidities, Activities of Daily Living (ADLs) and Instrumental Activities of Daily Living (IADLs). Patients were monitored for adverse events and whether a Geriatrician review occurred. Outcomes assessed at three, six and twelve months included increased dependency, falls, confusion, readmission to hospital, transfer to a Residential Aged Care Facility and death. Results 217 patients were recruited between 2018 and 2023 across the three hospitals. At index admission, 32 (14.7%) patients were frail and another 28 (12.9%) were near frail. Geriatrician review was more likely for frail patients and there were similar rates of inpatient complications for both frail and non-frail patients. Frailty at index admission was associated with increased dependency, falls, readmission and confusion at three, six and twelve months and with an increased risk of death at three and six months. Conclusions Frailty was associated with delayed adverse outcomes up to 12 months following admission for serious trauma in older people at three major Australian trauma services. Assessment of frailty on admission may be useful in stratifying outcome risk for older patients. Further research into frailty interventions and pathways is recommended.