Children’s emergent mathematics skills are important predictors of their later achievements. We analyze and present the effects of a large-scale school readiness intervention in preschools in Denmark. We thereby provide evidence on the effects of a broadly based professional development program (with supporting materials) implemented in a universal childcare system. Estimates of the effects were obtained from a cluster-randomized controlled trial on a large sample of more than 5000 children. The children’s emergent mathematics skills were assessed using a formal validated test (Tools for Early Assessment in Math) before and after the intervention. The results showed that the intervention on average had significantly positive effects for the participating children. Only children of non-Western origin benefitted more than other children from the intervention. In addition, the effects were moderated by exposure and by child-staff-ratio; stronger effects were associated with higher exposure and with lower child-staff-ratio.