Paternal support is often linked to lower levels of maternal distress. However, this link is less established among the increasing numbers of Mexican-American families with a romantic partner social (RPS) father, that is, mothers' partners who are not formally identified as stepfathers. This study applied a bioecological systems framework to test linkages between RPS father support and maternal depression and parenting stress above and beyond ecological stressors, and to consider whether nonresident biological father support and general instrumental support moderate this link. Using data from the Fragile Families and Child Wellbeing Study, we analyze a subsample of Mexican-American mothers (N = 76) with three-year-olds, who are involved in a relationship with a RPS father and maintain contact with the nonresident biological father. Findings indicate that mothers who reported greater support from RPS fathers also reported lower depressive symptomatology when they also reported greater support from nonresident biological fathers or reported being in a recent relationship with the RPS father; mothers from more established relationships reported more depressive symptoms. However, mothers with lower perceived instrumental social support reported high maternal depressive symptoms, even while receiving support from RPS fathers. Neither source of support significantly predicted maternal parenting stress. Overall, our results reveal complex, interactive associations between these combined sources of support and maternal mental health in these increasingly common family structures.