Therapeutic decision‐making for older patients with stage IV non‐small‐cell lung cancer (NSCLC) with no identifiable activating mutation is complex. In this prospective study, we evaluated the usefulness of geriatric assessment (GA) in identifying frail patients. Stage IV NSCLC patients ≥70 years of age were evaluated with GA and classified according to this evaluation into three different groups: fit, vulnerable and frail. Classifications based on GA, treatment decision, toxicity and overall survival were analysed. In total, 93 patients were included. Median age was 76 (70–92) years and 90% were men. Most patients had performance status (PS) 0 or 1 (82%), unrelated to their GA (p = 0.006). GA groups were associated with overall survival (p = 0.000), treatment decision (p = 0.0001), and toxicity (p = 0.0001). Chemotherapy was delivered to 100% of fit patients, to 48% of vulnerable patients, and to only 8% of frail patients (p = 0.000). Toxicity was higher in vulnerable patients than in fit individuals (p = 0.000). Multivariable analysis showed PS (p = 0.001), active treatment (p < 0.001) and GA group (p = 0.001) to be prognostic factors related to survival. Our results suggest that GA identified patients with poor natural prognosis.