Objective: To investigate the associations between frailty status and the risks of mortality and vascular complications in adults with type 1 diabetes (T1D). Methods: This prospective cohort study was based on the UK Biobank (UKB) and included participants with T1D at baseline. Frailty status was assessed using physical frailty (PF) and the frailty index (FI). The primary outcomes included all-cause mortality, T1D-related and complication-related mortality, and incident macrovascular and microvascular complications. Cox proportional hazards regression models were used to evaluate the associations between frailty status and each outcome. Dose–response analyses, subgroup analyses, and multiple sensitivity analyses were further conducted. Results: A total of 1,571 participants with T1D at baseline were included in the mortality analyses, with a median follow-up of 13.4 years; among them, 1,207 participants without baseline vascular complications were included in the complication analyses. Based on PF and FI, 12.7% and 28.5% of participants were classified as frail, respectively. After multivariable adjustment, compared with non-frail participants, frail individuals had a significantly higher risk of all-cause mortality (PF: hazard ratio (HR) 2.76, 95% confidence interval (CI) 1.92–3.98; FI: HR 2.71, 95% CI 1.71–4.31). The risk of T1D-related and complication-related mortality was also markedly increased among frail participants (PF: HR 4.02, 95% CI 2.21–7.28; FI: HR 4.14, 95% CI 1.73–9.89). Among participants without baseline vascular complications, frailty was significantly associated with higher risks of incident macrovascular complications (PF: HR 2.17, 95% CI 1.46–3.22; FI: HR 2.39, 95% CI 1.61–3.54) and microvascular complications (PF: HR 1.66, 95% CI 1.18–2.32; FI: HR 2.05, 95% CI 1.47–2.86). Dose-response analyses showed monotonic increases in the risks of multiple outcomes with increasing frailty severity, whereas a non-linear association was observed between PF scores and the risk of microvascular complications. Subgroup analyses showed that the association between FI and all-cause mortality was more pronounced among participants with lower systolic blood pressure (SBP) or lower body mass index (BMI), whereas the predictive effect of PF was relatively consistent across subgroups. Sensitivity analyses yielded results generally consistent with the main analyses. Conclusion: In adults with T1D, frailty was significantly associated with increased risks of mortality and incident vascular complications. Frailty assessment may help identify individuals at higher risk of poor prognosis and provide additional support for risk stratification and management in this population.