Recognizing that postpartum mothers’ organizational reentry is fraught with physical, emotional, and psychological challenges, we explored the specific behaviors that coworkers and managers can enact to support and advocate for working mothers during their reentry process—behaviors we conceptualize as postpartum allyship. In Study 1, we adopted a qualitative approach to gain insight into the forms of allyship that working mothers found valuable. We then build upon these findings in Study 2 by developing and validating a scale of postpartum allyship. Finally, in Study 3, integrating emergent themes from our qualitative data with tenets of the social cognitive model of career self-management (Lent and Brown, 2013, Lent and Brown, 2019), we use our newly-developed measure in a time-lagged study focused on the cognitive, affective, and behavioral impact of postpartum mothers’ experiences of allyship. Results indicated that postpartum allyship experiences bolster work-motherhood self-efficacy and reduce guilt which, in turn, yield important implications for working mothers’ turnover intentions, work-family capital, and postpartum depressive symptoms. Combined across our studies, the current research illuminates the critical impact of allies’ support and advocacy for postpartum mothers during reentry.