It has long been known that marriage is a critical correlate of mental health, primarily through relationship quality and support from partner. However, in contexts where couples struggle to maintain a healthy relationship and marriage is an increasingly protracted process, the benefits of marriage for women's mental health are far from assured. In this analysis, we draw on survey and qualitative data from a longitudinal study in two low-income communities in Nairobi, Kenya, to unpack the complex relationships among the conditions of marriage, kinship support, and the risk of depression among mothers with young children. Using cross-lagged, mediation, and growth models, we find some support for the benefits of union formalization for mothers' mental health explained primarily through relationship satisfaction. Qualitative data help explain the pathways through which these benefits accrue but also highlight ways in which the process of formalizing a union can undermine mothers' mental health.