Importance Metastatic non–small cell lung cancer (mNSCLC) has very high mortality rates, and comprises approximately half of new cases of lung cancer; however, highly effective and better tolerated treatments have become available in recent decades. Nevertheless, population-level treatment of mNSCLC is poorly characterized in the era of rapid treatment advances. Objective To characterize treatment rates, trends, and factors associated with treatment of mNSCLC. Design, Setting, and Participants This population-based study used linked Surveillance Epidemiology and End Results (SEER) and Medicare claims data and the analysis included patients 65 years and older diagnosed with mNSCLC from January 2006 to December 2021. Data were analyzed from October 2025 to February 2026. Exposures Sociodemographic variables, comorbidity burden, histologic type, referral to subspecialist, enrollment in Medicare Part D, and biomarker testing. Main Outcomes The primary outcome was receipt of systemic treatment. Statistical analyses included summary statistics and a competing risk proportional hazards model for receipt of systemic treatment. Results Of 254 611 patients with mNSCLC, the cohort median (IQR) age was 73 (68-80) years, with 133 635 (52.5%) male individuals; a total of 9512 (3.7%) were Asian, 26 546 (10.4%) were Black, 4553 (1.8%) were Hispanic, 205 381 (80.7%) were White, and 8619 (3.4%) were another or unknown race. A total of 119 197 patients (46.8%) ever received systemic treatment. Of the 100 367 (39.8%) who died within 90 days of diagnosis, 13.2% were treated compared with 69% of those surviving more than 90 days. The treated proportion increased only slightly between 2006 and 2021. In a competing risk model, referral to oncology specialists was associated with treatment (hazard ratio [HR], 2.5; 95% CI, 2.41-2.67; P &lt; .001) which corresponded to a 30.3% greater cumulative incidence of treatment at 180 days (CIF180) compared with those without a referral. Similarly, those with biomarker testing had a 17.8% greater CIF180, whereas those older than 80 years had a 15.4% lower CIF180 compared to those aged 65 to 69 years. Patients with NSCLC not otherwise specified histologic findings had a 12.8% lower CIF180 compared with those with adenocarcinoma histologic findings. Other factors associated with significant but smaller differences in receipt of treatment included comorbidity burden, marital status, Medicare Part C or Part D coverage, rurality, and race and ethnicity. Conclusions and Relevance In this cohort study of older adults with mNSCLC, despite advances in therapy in recent decades, almost half of patients never received systemic therapy, and the proportion treated only minimally improved over time. Approximately one-fifth of those with the most favorable clinical profiles did not receive systemic therapy.