ImportanceDuring an ongoing longitudinal cohort study, a casino opening created a natural cash transfer experiment. Some participating families received income supplements, and others did not. The children in this study are now adults.ObjectiveTo assess the long-term outcomes of family income supplements received in childhood.Design, Setting, and ParticipantsThis community-representative longitudinal cohort study set in western North Carolina assessed 1266 participants aged 9, 11, and 13 years at intake up to 11 times up to age 30 years from January 1993 to December 2015. Data were analyzed from January to December 2021.ExposuresIn 1996, a southeastern American Indian tribe implemented a cash transfer program of approximately $5000 annually per person for tribal members. Participants were compared on whether their family ever received the cash transfers (American Indian vs non–American Indian), the duration of the transfers, and annual amount based on the number of parents.Main Outcomes and MeasuresParticipants were followed up at ages 25 and 30 years to assess mental health symptoms, substance use symptoms, and functional outcomes (physical health, risky or illegal behaviors, and financial and social functioning).ResultsOf 1266 included participants, 320 (25.3%) were American Indian and 581 (49.7%) were female. Participants whose families received cash transfers during childhood reported fewer anxiety symptoms (relative risk [RR], 0.33; 95% CI, 0.25-0.44), depressive symptoms (RR, 0.51; 95% CI, 0.42-0.62), and cannabis symptoms (RR, 0.47; 95% CI, 0.27-0.82). They also reported improved physical health (RR, 0.66; 95% CI, 0.55-0.80) and financial functioning (RR, 0.78; 95% CI, 0.67-0.89) and fewer risky or illegal behaviors (RR, 0.57; 95% CI, 0.46-0.72) compared with those who did not receive the cash transfer. This pattern was supported by a series of heterogeneity analyses in which children whose families received the transfers for the longest duration and whose families received the largest transfer (due to having multiple American Indian parents) had the lowest levels of symptoms and the highest levels of functioning.Conclusions and RelevanceIn this natural experiment, a family cash transfer in childhood was associated with positive adult functioning 20 years later. The findings support programs like the child tax credit or universal basic income that provide cash directly to families with children.