Background Frailty is common in patients with gastrointestinal malignancies, and is a marker of vulnerability in elderly patients, potentially contributing to poorer surgical outcomes in this group. We aimed to explore the impacts of age and frailty on postoperative mortality and ‘failure to rescue’ (FTR). Methods Patients aged over 65 years undergoing gastrointestinal or hepatopancreatobiliary cancer resection from 2005-2020 were identified from the New Zealand Cancer Registry and National Minimum Dataset. FTR was defined as the 90-day mortality rate amongst patients with any of 19 postoperative complications. The Hospital Frailty Risk Score and C3 Comorbidity Index were used to measure frailty and comorbidity respectively. Restricted cubic spline analysis was used to explore risk-adjusted effects of age, comorbidity, and frailty on outcomes. Results In total, 21,729 patients were included, with a mean age of 75±7 years. Most patients were undergoing colorectal (84%) or gastric (6%) cancer resection. Frailty was identified in 12% of patients (10% intermediate-risk, 2% high-risk). A high risk of frailty was associated with increased postoperative mortality (18% vs 5%) and FTR (26% vs 11%). Restricted cubic spline analysis demonstrated independent and additive impacts of age and frailty on postoperative mortality and FTR, with a smaller increase in the risk of postoperative complications. In comparison, comorbidity had a minor impact on risk-adjusted mortality, FTR, and complications. Conclusion Frailty and age are independent predictors of poorer outcomes for elderly patients undergoing major cancer surgery. Higher risks of mortality in this group are predominantly mediated by an increased risk of FTR following complications.