The transition to elementary school is an important but often challenging life event not only for children but also for their parents in Korea. Many Korean parents seek information and support from their personal networks while helping their children prepare for elementary school. In this study, we aimed to examine the relationships among the social support mothers actually receive, mothers’ parenting competency, and children’s school readiness with a special attention given to the mediating role of mothers’ parenting competency between social support and school readiness. We collected data in February 2017 from 368 mothers who were preparing to send their first child to elementary school in March 2017. After verifying the measurement model, we conducted structured equation modeling to test our conceptual model. We found that mothers’ higher levels of social support were related to higher parenting competency, which in turn, was associated with their children’s higher levels of readiness for elementary school. Bootstrapping results indicated that parenting competency significantly mediated the relation between the mothers’ received social support and their children’s school readiness. Our results suggest that efforts to strengthen the social relationships of mothers whose children are entering elementary school facilitate the mothers’ parenting competency, resulting in better school readiness for their children.