More than 50 000 youths are incarcerated in the United States on any given day, and youth incarceration has been linked to lasting adverse outcomes, including early mortality. Improving our understanding of the factors associated with early mortality among incarcerated youths can inform appropriate prevention strategies. To examine mortality rates and causes of death among youths previously incarcerated in the juvenile legal system. This retrospective longitudinal population-based cohort study compared mortality rates between youths aged 11 to 21 years incarcerated from 2010 to 2017 with same-aged nonincarcerated Medicaid-enrolled youths in the state of Ohio. Data from January 2017 to December 2019 were collected from juvenile incarceration, Medicaid, and death certificate information in Ohio. Incarceration in the state of Ohio's juvenile legal system. Number, characteristics, and causes of deaths. Poisson regression incidence rate ratios (IRRs) compared mortality rates between previously incarcerated and Medicaid-enrolled youths. Among 3645 incarcerated youths, 3398 (93.2%) were male, 2155 (59.1%) Black, 1307 (35.9%) White, and 183 (5.0%) other race and ethnicity. Overall, 113 youths (3.1%) died during the study period. Homicide was the leading cause of death in formerly incarcerated youths (homicide: 63 [55.8%]; legal intervention [ie, death due to injuries inflicted by law enforcement]: 3 [2.7%]). All-cause mortality rates were significantly higher among previously incarcerated youths than Medicaid-enrolled youths (adjusted IRR [aIRR], 5.91; 95% CI, 4.90-7.13) in every demographic subgroup. Compared with Medicaid-enrolled youths, mortality rates for previously incarcerated youths were highest for homicide (aIRR, 11.02; 95% CI, 8.54-14.22), overdose (aIRR, 4.32; 95% CI, 2.59-7.20), and suicide (aIRR, 4.30; 95% CI, 2.22-8.33). Formerly incarcerated Black youths had a significantly higher risk of homicide (aIRR, 14.24; 95% CI, 4.45-45.63) but a lower risk of suicide (aIRR, 0.18; 95% CI, 0.04-0.89) and overdose (aIRR, 0.31; 95% CI, 0.10-0.99) than White youths who were incarcerated. Previously incarcerated youths aged 15 to 21 years were significantly more likely to die than youths aged 22 to 29 years, irrespective of cause of death (aIRR for youths aged 22-29 years, 0.09; 95% CI, 0.06-0.14). In this study, youths with a history of incarceration were significantly more likely to experience early mortality compared with nonincarcerated Medicaid-enrolled youths. Delinquency and violence prevention strategies that incorporate a culturally informed approach and consider sex and developmental level are critical.