Prior theoretical and empirical studies have linked the first 3 years of children's life with later life outcomes. One primary explanation is the critical role these experiences play in children's early brain development (including their early language and cognitive abilities) and subsequent schooling achievement. The current study is a registered report with two complementary research objectives: (1) examine to what extent stimulating and responsive interactions with mothers and nonparental caregivers during the first 3 years of life show additive or synergistic associations with key socioeconomic outcomes in adulthood (i.e., educational attainment, salary, and employment status), and (2) examine to what extent academic skills (i.e., mathematics, vocabulary, and literacy) during childhood and adolescence mediate these associations. The sample included 1364 individuals from a birth cohort study who were followed until age 26. Mother-child interactions had positive associations with educational attainment and negative associations with full-time employment, though no synergistic associations were found for the socioeconomic outcomes. In addition, the indirect effects of mother-child interactions on educational attainment through mathematics were strongest for children with less stimulating and responsive caregiver-child interactions. Potential implications of these findings for developmental theory are discussed.