Importance Identifying early childhood behavioral problems associated with economic success/failure is essential for the development of targeted interventions that enhance economic prosperity through improved educational attainment and social integration. Objective To test the association between kindergarten teacher–rated assessments of inattention, hyperactivity, opposition, aggression, and prosociality in boys with their employment earnings at age 35 to 36 years as measured by government tax return data. Design, Setting, and Participants A 30-year prospective follow-up study analyzing low socioeconomic neighborhoods in Montreal, Quebec, Canada. Boys aged 5 to 6 years attending kindergarten in low socioeconomic neighborhoods were recruited. Teacher-rated behavioral assessments were obtained for 1040 boys. Data were collected from April 1984 to December 2015. Analysis began January 2017. Main Outcomes and Measures Mixed-effects linear regression models were used to examine the association between teacher ratings of inattention, hyperactivity, opposition, aggression, and prosociality at age 6 years and individual earnings obtained from government tax returns at age 35 to 36 years. The IQ of the child and family adversity were adjusted for in the analysis. Results Complete data were available for 920 study participants (mean age at follow-up was 36.3 years). Mean (SD) personal earnings at follow-up were $28 865.53 ($24 103.45) (range, $0-$142 267.84). A 1-unit increase in inattention (mean [SD], 2.66 [2.34]; range, 0-8) at age 6 years was associated with decrease in earnings at age 35 to 36 years of $1295.13 (95% CI, −$2051.65 to −$538.62), while a unit increase in prosociality (mean [SD], 8.0 [4.96]; range, 0-20) was associated with an increase in earnings of $406.15 (95% CI, $172.54-$639.77). Hyperactivity, opposition, and aggression were not significantly associated with earnings. Child IQ was associated with higher earnings and family adversity with lower earnings in all models. A 1-SD reduction in inattention at age 6 years was associated with a theoretical increase in annual earnings of $3040.41, a similar magnitude to an equivalent increase in IQ. Conclusions and Relevance Teacher ratings of inattention and prosociality in kindergarten boys from low socioeconomic neighborhoods are associated with earnings in adulthood after adjustment for hyperactivity, aggression, and opposition, which were not associated with earnings. Interventions beginning in kindergarten that target boys’ inattention and enhance prosociality could positively impact workforce integration and earnings.