ImportanceMedicaid expansion under the Patient Protection and Affordable Care Act is associated with gains in health insurance coverage, earlier stage diagnosis, and improved survival among patients with cancer.ObjectiveTo examine the association of Medicaid expansion with changes in early mortality among adults undergoing surgical resection of non–small cell lung cancer (NSCLC), a setting in which access to care is a major determinant of survival.Design, Setting, and ParticipantsThis cohort study used the National Cancer Database to identify 14 984 adults 45 to 64 years of age who underwent surgical resection of NSCLC between 2008 and 2019. Analysis was conducted between March 28, 2021, and September 1, 2023.ExposureState of residence Medicaid expansion status.Main Outcomes and MeasuresDescriptive statistics were used to compare study population characteristics by Medicaid expansion status of patients’ state of residence. Difference-in-differences analyses were used to evaluate the association between Medicaid expansion and postoperative mortality before implementation of the ACA (2008-2013) vs after (2014-2019).ResultsAmong 14 984 adults included, the mean (SD) age was 56.3 (5.1) years, 54.6% were women, and 62.1% lived in Medicaid expansion states. Both 30-day (from 0.97% to 0.26%) and 90-day (from 2.63% to 1.32%) postoperative mortality decreased from before the ACA to after among patients residing in Medicaid expansion states (both P < .001) but not in nonexpansion states (30-day mortality before the ACA, 0.75% vs after the ACA, 0.68%; P = .74; and 90-day mortality before the ACA, 2.43% vs after the ACA, 2.20%; P = .57), leading to a difference-in-differences of −0.64 percentage points (95% CI, −1.19 to −0.08; P = .03) for 30-day mortality and −1.08 percentage points (95% CI, −2.08 to −0.08; P = .03) for 90-day mortality. The difference-in-differences for in-hospital mortality was not significant (P = .34) between expansion states (1.41% before the ACA to 0.77% after the ACA; 0.63 percentage point decrease; P = .004) and nonexpansion states (1.49% before the ACA to 1.20% after the ACA; 0.30 percentage point decrease; P = .29).Conclusions and RelevanceIn this cohort study of patients with NSCLC, Medicaid expansion was associated with declines in 30- and 90-day postoperative mortality following hospital discharge. These findings suggest that Medicaid expansion may be an effective strategy for improving access to care and cancer outcomes in this population.