Background: Patients with metastatic non-small cell lung cancer (NSCLC) often experience severe eating disorders and other problems because of rapid disease progression and various therapies, which significantly affect their quality of life (QoL). Aims: This study aimed to assess the nutritional status and QoL of patients with NSCLC at the start of immunotherapy and 3 months later. Methods: This study used a prospective, observational, consecutive sampling approach and was carried out between July 2018 and August 2019. The convenience sample comprised 60 patients treated in the oncology ward and the day care department of a public hospital in Greece. Their nutritional status was assessed using the Mini Nutritional Assessment questionnaire and their QoL using the European Organization for Research and Treatment of Cancer QLQ-C30 questionnaire and its QLQ-LC13 module. They were evaluated at the start of immunotherapy (phase I) and 3 months later (phase II). Findings: Of the initial 60 patients, 25% had died and 23.3% declined to participate by phase II. Of the 31 patients (51.7%) who completed the study, 64.5% had been assessed as malnourished at phase I; at phase II, this had decreased to 32.3%, so nutritional status appeared to have significantly improved. There were also significant improvements between phases I and II in global health status/QoL, as well as in the physical, roles, emotional and social dimensions. Additionally, all nine symptoms of QLQ-C30 significantly decreased, while the QLQ-LC13 showed significant improvements in dyspnoea, alopecia and pain in parts of the body other than the chest, arm or shoulder. Conclusion: Metastatic NSCLC patients' QoL and nutritional status significantly improved after 3 months' immunotherapy.