We examine the causal effect of health insurance on mortality using the universe of low-income adults, a dataset of 37 million individuals identified by linking the 2010 Census to administrative tax data. Our methodology leverages state-level variation in the timing and adoption of Medicaid expansions under the Affordable Care Act (ACA) and earlier waivers and adheres to a preregistered analysis plan, a rarely used approach in observational studies in economics. We find that expansions increased Medicaid enrollment by 12 percentage points and reduced the mortality of the low-income adult population by 2.5 percent, suggesting a 21 percent reduction in the mortality hazard of new enrollees. Mortality reductions accrued not only to older age cohorts, but also to younger adults, who accounted for nearly half of life-years saved due to their longer remaining lifespans and large share of the low-income adult population. These expansions appear to be cost-effective, with direct budgetary costs of $5.4 million per life saved and $179,000 per life-year saved falling well below valuations commonly found in the literature. Our findings suggest that lack of health insurance explains about five to twenty percent of the mortality disparity between high- and low-income Americans. We contribute to a growing body of evidence that health insurance improves health and demonstrate that Medicaid’s life-saving effects extend across a broader swath of the low-income population than previously understood.