The present study examined whether and how socioeconomic status (SES) predicts school achievement in science, technology, engineering, and math (STEM) using structural equation modeling and data from the National Institute of Child Health and Human Development Study of Child Care and Youth Development. The present inquiry addresses gaps in previous research linking SES and STEM achievement in high school. Results indicate that maternal education predicts the child's early environment, which itself predicts the development of executive function (EF) and language, and thereby, STEM achievement. Moreover, children's language ability and EF development influenced higher-order cognitive skills, such as relational reasoning, planning, and basic calculation skills. However, only relational reasoning strongly predicted high school math and science achievement, suggesting that relational reasoning, but not planning and calculation skills, was central to STEM thinking and learning.