Mother-child cohesion, reflecting the emotional connection between mothers and children, is a protective factor for individuals’ mental health. The Modified Operations Triad Model suggests that children and mothers hold either congruent or incongruent views of their perceived cohesion, which reflects family functioning and is an indication of future mental health outcomes for family members. Despite increasing research on the impact of (in)congruence in perceived family functioning on children’s mental health, few studies concurrently address the mental health of both mothers and children, overlooking their interdependence. This longitudinal study with multi-informant reports explored the long-term and developmental effects of (in)congruence in perceived mother-child cohesion on informants’ depressive symptoms. A total of 577 families participated at the first time point, comprising 577 children (52.34% girls, Mage = 10.11 years) and 577 mothers (Mage = 37.32 years). The Dyadic Response Surface Analysis revealed that mother-child congruence at high levels of cohesion (versus low levels) predicted fewer depressive symptoms in mothers and children after three and nine months, as well as decreased depressive symptoms over nine months. Children who reported higher levels of cohesion than their mothers experienced fewer depressive symptoms after three months. Mothers with higher levels of cohesion than their children reported fewer depressive symptoms after three months. The current findings emphasize the importance of identifying and resolving discrepancies in perceived mother-child cohesion between mothers and children to promote a healthy family environment.