The Affordable Care Act (ACA) expanded health care access in the United States. This study examines the long-term impact of the ACA on private health insurance enrollment using National Health Interview Survey (NHIS) data. A repeated cross-sectional study using NHIS data from 2015 to 2022 was analyzed. Given the repeal of the ACA's individual mandate in 2019, stratified analyses compared Marketplace enrollment before (2015, 2018) and after (2019, 2022) the repeal. The study included US adults aged 26-64 years. Unadjusted enrollment rates were compared across age, sex, race/ethnicity, social determinants of health (SDOH), chronic conditions, body mass index, and smoking. Multivariable logistic regression assessed enrollment trends and associated factors. Marketplace enrollment increased by 1.4 percentage points post-mandate (