The vulnerability and instability of low-income mothers situated in a context with a weak public safety net make informal social support one of few options many low-income mothers have to meet basic needs. This systematic review examines (a) social support as an empirical construct, (b) the restricted availability of one important aspect of social support—informal perceived support, hereafter informal support—among low-income mothers, (c) the role of informal support in maternal, economic, parenting, and child outcomes, (d) the aspects of informal support that influence its effects, and (e) directions for future research. Traditional systematic review methods resulted in an appraisal of 65 articles published between January 1996 and May 2017. Findings indicated that informal support is least available among mothers most in need. Informal support provides some protection from psychological distress, economic hardship, poor parenting practices, and poor child outcomes. To promote informal support and its benefits among low-income families, future research can advance knowledge by defining the quintessential characteristics of informal support, identifying instruments to capture these characteristics, and providing the circumstances in which support can be most beneficial to maternal and child well-being. Consistent measurement and increased understanding of informal support and its nuances can inform intervention design and delivery to strengthen vulnerable mothers’ informal support perceptions thereby improving individual and family outcomes.