It is well attested that high socio-economic status (SES) is associated with larger vocabulary size estimates in young children. This has led to growing interest in identifying associations and mechanisms that may contribute to this relationship. In this study, parent-child reading behaviors were investigated in relation to vocabulary size in a large-scale study of linguistically and socio-economically diverse families. This study sampled 902 infants in Singapore, a multilingual society. Both single-language (dominant and non-dominant) and dual-language vocabulary size estimates were obtained and related to family SES, demographic details, and home literacy activities. Results demonstrated that both single-language (dominant and non-dominant) and dual-language infant vocabulary size estimates were predicted by parental education levels. Further analyses revealed that parent-child book reading activities mediated the relationship between parental education and infant vocabulary size. Findings suggest that shared book reading may narrow effects of socio-economic disparities on early language development.