Research indicates that mathematical understanding before formal schooling influences later attainment. This study investigated the mathematical activities of preschool-aged children in Japan, a neglected topic despite the high mathematical attainment of Japanese children. Preschool activities were investigated using surveys, observations, and staff interviews; activities at home and use of non-preschool educational providers were investigated using surveys of parents. The results showed that preschools overwhelmingly followed the government curricula, seeking to develop children’s interest in and sense of number, quantity, and shape through activities embedded in play and daily life; more directed teaching using workbooks was rare. Many children frequently engaged in informal mathematics-related activities at home. An unexpectedly high proportion of children engaged with learning materials from non-preschool educational providers. Results were broadly consistent across the two locations studied, despite the contrasting attainment of children in these areas in national mathematics tests. The results demonstrate that preschools in different parts of Japan avoid directed mathematical teaching. Further research is needed into the development of the mathematical understanding of preschool-aged children in Japan, especially in relation to non-preschool educational providers.