Parents are their children's most influential educators, and their joint activities can influence these children's early learning. Past studies with small, non-representative samples do not show a consistent link between early numeracy activities at home and children's mathematics achievement. Specifically, whether or how "early numeracy activity at home" (ENAH) enhances mathematics learning in upper primary school remains an open question. This study tests this link, its precursors (home resources for learning, gender), and its possible mechanisms (including early numeracy skills, mathematics interest, and mathematics self-efficacy) on a representative sample of 3,600 Hong Kong fourth-grade children, with a multilevel path analysis. The results showed that ENAH was linked to both early numeracy and fourth-grade mathematics achievement, and did not support the substitution hypothesis (that other factors such as school lessons substitute for ENAH). The results also support two ENAH mechanisms. Children's early numeracy and mathematics self-efficacy both partially mediated the link between ENAH and children's later mathematics achievement. After including these explanatory variables in the model, ENAH still retained a significant direct link to fourth-grade mathematics achievement, suggesting that ENAH also operates through one or more other mechanisms. Lastly, boys and children in families with more home resources for learning were more likely than other children to participate in ENAH.