Poverty is associated with poorer mental health in early adulthood. Cash transfers (CTs) have been shown to improve child health and education outcomes, but it is unclear whether these effects may translate into better mental health outcomes as children reach young adulthood. Using a quasi-experimental approach that exploits variation across countries in the timing of national CT programme introduction, we examine whether longer exposure to CTs during childhood (0-17 years) reduces depressive symptoms in early adulthood (18-30 years). Based on harmonized data from Colombia, Mexico, and South Africa (N = 14 431), we applied logistic regression models with country and birth-cohort fixed effects to estimate the impact of cumulative years of CT exposure on mental health, educational attainment, and employment outcomes. Our findings indicate that each additional year of CT exposure during childhood is associated with a 4% reduction in the odds of serious depressive symptoms in early adulthood [odds ratio (OR) = 0.96, 95% confidence intervals (CIs): 0.93, 0.98]. We find no consistent effect of years of exposure on completion of secondary school (OR = 1.01, 95% CIs: 0.99, 1.03) and a negative effect on the probability of employment in early adulthood (OR = 0.90, 95% CIs: 0.88, 0.91). These results suggest that longer exposure to CTs may contribute to modest but meaningful reductions in population-level depressive symptoms during early adulthood.