OBJECTIVES: To test associations between onset of formal child care (in infancy or as a toddler), high school graduation, and employment earnings from ages 18 to 35 years. METHODS: A 30-year prospective cohort follow-up study, with linkage to government administrative databases (N =3020). Exposure included formal child care, if any, by accredited caregivers in centers or residential settings at ages 6 months and 1, 1.5, 2, 3, and 4 years. A propensity score analysis was conducted to control for social selection bias. RESULTS: Of 2905 participants with data on child care use, 59.4% of male participants and 78.5% of female participants completed high school by age 22 to 23. Mean income at last follow-up (n = 2860) was $47 000 (Canadian dollars) (SD = 37 700) and $32 500 (SD = 26 800), respectively. Using group-based trajectory modeling, we identified 3 groups: formal child care onset in infancy (∼6 months), formal child care onset as a toddler (after 2.5 years), and never exposed. After propensity score weighting, boys with child care started in infancy had greater odds of graduating than those never exposed (odds ratio [OR] 1.39; 95% confidence interval [CI]: 1.18–1.63; P < .001). Boys attending child care had reduced odds of low income as young adults (infant onset: OR 0.60 [95% CI: 0.46–0.84; P < .001]; toddler onset: OR 0.63 [95% CI: 0.45–0.82; P < .001]). Girls’ graduation rates and incomes revealed no significant association with child care attendance. CONCLUSIONS: For boys, formal child care was associated with higher high school completion rates and reduced risk of adult poverty. Benefits for boys may therefore extend beyond school readiness, academic performance, and parental workforce participation.