This study examines high school diploma or equivalent (HS/E) attainment by mothers who had a nonmarital birth (“unmarried mothers”) and the associations between state‐level Temporary Assistance for Needy Families (TANF) policies and their postnatal HS/E attainment. Using an analytic sample of 1154 unmarried mothers without HS/E from the restricted‐use Future of Families and Child Wellbeing Study, we tracked postnatal HS/E attainment patterns for 15 years, and conducted discrete‐time event history analysis with mixed effects to test the relationships between states' TANF policies and postnatal HS/E attainment. Only 35.1% of the sample attained HS/E after childbirth, while nearly half of mothers who attained HS/E were not able to do so within 3 years of childbirth. A $100 higher maximum monthly benefit amount is associated with 86.1% higher odds of postnatal HS/E attainment, indicating the need to consider increasing TANF benefit amounts as a means to promote educational attainment among unmarried mothers with educational disadvantages.