This study examined maternal support of children's math learning at 36 months (n = 140) as a mediator of the association between early childhood family income and children's counting and calculation skills at 4½ and 6-7 years. Family income was associated with this measure of children's math school readiness, but the association was almost entirely indirect and mediated by observed maternal support of children's numerical skills at 36 months. Maternal support of children's spatial concept and general learning (cognitive stimulation and sensitivity) were not significant mediators of this association. Results suggest that income-based gaps in counting and calculation skills at school entry may be due in part to the constraints that low family income places on early numerical learning support.