Maternal psychological well-being has a significant impact on both parents’ health and children’s developmental outcomes. While understanding the family and socio-contextual factors that lead to healthy psychological adjustment for mothers is crucial, little is known about the mechanisms responsible for transmitting early marital quality to later psychological well-being. The proposed study examined the coparenting relationship as an explanatory mechanism in pathways from marital quality to mothers’ stress and depressive symptoms. Data were drawn from an ethnically and socioeconomically diverse sample of parents who participated in the Supporting Healthy Marriage (SHM) Project. Marital satisfaction and conflict were assessed using surveys at baseline study enrollment (Wave 1; N=6231), coparental conflict was assessed using surveys and semi-structured observations, respectively, at the 12-month follow-up visit (Wave 2; N=5114), and mothers reported on stress and depressive symptoms at 30-month follow-up (Wave 3; N=4864). Path analyses for the sample as a whole indicated that higher initial levels of marital satisfaction and lower levels of conflict were linked to less coparental conflict and fewer maternal depressive symptoms. Significant indirect effects indicated that paths from marital quality to both maternal stress and depressive symptoms were mediated through coparental conflict. Exploratory analyses of multi-group models revealed that all four significant indirect pathways held for US-born parents, but only one was significant for Hispanic immigrant parents. Further, one pathway was moderated by group membership, such that higher marital satisfaction was more strongly related to reduced coparental conflict in US-born relative to Hispanic immigrant parents. Findings provide support for the coparenting relationship as a mechanism by which marital functioning affects maternal psychological well-being, while also documenting unique pathways for US-born parents and Hispanic immigrant parents.