Importance Specifying the association between childhood behaviors and adult earnings can inform the development of screening tools and preventive interventions to enhance social integration and economic participation. Objective To test the association between behaviors at age 6 years and employment earnings at age 33 to 35 years. Design, Setting, and Participants This study obtained data from the Quebec Longitudinal Study of Kindergarten Children, a population-based sample of boys and girls (n = 3020) born in 1980 or 1981 in Quebec, Canada, and followed up from January 1, 1985, to December 31, 2015. The data included behavioral ratings by kindergarten teachers when the children were aged 5 or 6 years and 2013 to 2015 government tax returns of those same participants at age 33 to 35 years. Data were analyzed from September 2017 to December 2018. Main Outcomes and Measures Mixed-effects linear regression models were used to test the associations between teacher-rated inattention, hyperactivity, aggression, opposition, anxiety, and prosociality at age 6 years and reported annual earnings on income tax returns at age 33 to 35 years. Participant IQ and family adversity were adjusted for in the analysis. Results The study included 2850 participants, with a mean (SD) age of 35.9 (0.29) years, of whom 1470 (51.6%) were male and 2740 (96.2%) were white. The mean (SD) personal earnings at follow-up were US $33 300 ($27 500) for men and $19 400 ($15 200) for women. A 1-unit increase in inattention score at age 6 years (males mean [SD], 2.47 [2.42] vs females mean [SD], 1.67 [2.07]) was associated with a decrease in annual earnings of $1271.49 (95% CI, −1908.67 to −634.30) for male participants and $924.25 (95% CI, −1424.44 to −425.46) for female participants. A combined aggression-opposition score (males mean [SD] 2.22 [2.52] vs females mean [SD], 1.05 [1.73]) was associated with a reduction in earnings of $699.83 (95% CI, −1262.49 to −137.17) for males only, albeit with an effect size roughly half that of inattention. A 1-unit increase in prosociality score (males mean [SD], 6.12 [4.30] vs females mean [SD], 7.90 [4.56]) was associated with an increase in earnings of $476.75 (95% CI, 181.53-771.96) for male participants only. A 1-SD reduction in inattention score at age 6 years would theoretically restore $3077 in annual earnings for male participants and $1915 for female participants. Conclusions and Relevance In this large population-based sample of kindergarten children, behavioral ratings at 5-6 years were associated with employment earnings 3 decades later, independent of a person’s IQ and family background. Inattention and aggression-opposition were associated with lower annual employment earnings, and prosociality with higher earnings but only among male participants; inattention was the only behavioral predictor of income among girls. Early monitoring and support for children demonstrating high inattention and for boys exhibiting high aggression-opposition and low prosocial behaviors could have long-term advantages for those individuals and society.