The current study investigated the extent to which a semi-structured block play intervention supported growth in mathematics and executive functioning for preschool children using a randomized controlled design. A secondary aim was to explore whether differential intervention effects emerged for children from various socioeconomic backgrounds, indicated by parental education level. Participants included 59 preschool children. Children ranged in age from 38 to 69 months (M = 55.20, SD = 7.17), and 56% were female. Results from regression models indicated that, although not statistically significant, children who participated in the intervention demonstrated greater gains in three mathematics skills (numeracy, shape recognition, and mathematical language) and two indicators of executive functioning (cognitive flexibility and a measure of global executive functioning) compared to children in a control group. Further, three significant interactions were found, suggesting that for numeracy, cognitive flexibility, and global executive functioning, children of parents with low educational attainment benefited the most from intervention participation. These findings provide preliminary evidence for the effectiveness of a semi-structured block play intervention for improving children’s school readiness and have implications for including intentional instruction using blocks in preschool classrooms.