This study investigated the effects of the Japanese Early Promotion Program (JEPP), which is based on the Infant Mental Health (IMH) program. The JEPP aims to promote mother-infant interactions by enhancing the mother’s ability to respond appropriately her child. Mothers in the JEPP group (n = 15) received support from IMH nurses in a pediatric clinic until their infants reached 12 months of age. The nurses provided positive feedback that emphasized strength of parenting, and assisted the mothers in understanding the construct of their infants. Mother-infant interactions and mother’s mental health status were assessed at intake (1–3 months), and at 6, 9, and 12 months of infants’ age. The JEPP group data were compared with cross-sectional data of the control group (n = 120). Although JEPP dyads were not found to be significantly different from the control group in general dyadic synchrony, both before and after intervention, JEPP mothers significantly improved their ability to understand their infant’s cues and to respond promptly. In the JEPP group, unresponsiveness to infants was reduced in mothers, while infants showed reduced passiveness and enhanced responsiveness to the mother. Furthermore, the intervention reduced the mothers’ parenting stress and negative emotions, thereby enhancing their self-esteem.