This article examined longitudinal relations among socioeconomic risk, maternal language input, child vocabulary, and child executive function in a large sample (N =1,009) recruited for a prospective longitudinal study. Two measures of maternal language input derived from a parent- child picture book task, vocabulary diversity and language complexity, showed variation by socioeconomic risk at child ages 15, 24, and 36 months. Maternal vocabulary diversity at child age 24 months and maternal language complexity at child age 36 months mediated the relation between socioeconomic risk and 48-month child executive function, independent of parenting sensitivity. Moreover, 36-month child vocabulary mediated the relation between maternal language input and child executive function. These findings provide novel evidence about mechanisms linking socioeconomic risk and child executive function. [This paper was written with FLP Key Investigators and was published in "Child Development" v90 n6 p2001-2018 2019.]