The postpartum period is a challenging transition period with almost one in ten mothers experiencing depression after childbirth. Perceived social support is associated with mental health. Yet empirical evidence regarding the causal effects of social support on postpartum mental health remains scarce. In this paper, we used a nationally representative panel data of women to examine causality between perceived social support and postpartum mental health. We used fixed-effect method and included dependent variable lags to account for past mental health condition before birth (i.e., the pre-pregnancy and prenatal periods). The study also used an instrumental variable approach to address endogeneity. We find a declining trend in postpartum mental health between 2002 to 2018. Our study also showed that past mental health (i.e., before childbirth) is positively correlated with postpartum mental health. A universal routine mental health screening for expectant and new mothers should remain a key priority to ensure mental wellbeing for the mothers and their infants.