The Vygotskyan perspective to school achievement suggests that parents play a role in mediating the relationship between students’ ability and their achievement. This study investigated parental influence on students’ cognitive ability which affected mathematics and language achievement within four primary schools in Hong Kong. The participants were 215 upper primary students, and their parents. Students were administered a working memory assessment and their achievement scores in mathematics and language were obtained from school records. Parents completed a questionnaire which profiled their cultural factors and SES. Data was Rasch analyzed and subjected to structural equation modeling. Findings showed that students’ cognitive ability was mediated by parental expectations and beliefs, as moderated by parental SES, in predicting mathematics and language achievement. This finding demonstrated that parental influence in students’ academic achievement extended to their cognitive ability, especially within the context of the four schools in Hong Kong, in line with the Vygotskyan supposition that parents are the significant other that frame, filter and interpret information to promote their children’s learning and development.