Aim This study examines the association between mental health symptoms (depression, anxiety, and stress) and quality of life (QoL) in lung cancer patients, stratified by the presence or absence of chronic obstructive pulmonary disease (COPD). It also identified predictors of physical and mental QoL. Background Lung cancer and COPD frequently coexist due to shared risk factors, such as smoking, which compounds physical and psychological burdens. Despite this, limited research has explored how this comorbidity affects mental health and QoL. Methods This cross-sectional study included 456 hospitalized lung cancer patients, of whom 235 had COPD. Depression, anxiety, and stress were assessed using the DASS-21, while QoL was measured using the SF-12. Multiple regression analyses were conducted to identify predictors of QoL. Results Patients with COPD reported significantly higher levels of depression, anxiety, and stress (p < .05). Stress was a significant negative predictor of physical QoL (adjusted R-2: 0.103, p < .001), while COPD and anxiety significantly negatively predicted mental QoL (adjusted R-2: 0.128, p < .001). Conclusions COPD exacerbates psychological distress and worsens QoL in lung cancer patients. Stress and anxiety significantly impair physical and mental QoL, underscoring the need for integrated care. Implications for practice Holistic care approaches that address stress and anxiety, promote smoking cessation, and provide personalized treatments are essential for improving QoL and outcomes in this vulnerable population.