The COVID-19 pandemic dramatically increased families’ childcare responsibilities, disproportionately affecting mothers and undermining their careers and wellbeing. Using two waves of interviews with 77 mothers of young children, we examined why mothers—especially in dual-earner, different-gender couples—took on more pandemic parenting. We found that pre-pandemic gendered structures of paid work led many families to lean on mothers by default. First, within-couple gaps in job types, pay, and work hours led some mothers to do more childcare to protect partners’ work as primary earners. Second, educational gaps within couples led some mothers to do more as the only parent able to work fully remote. Third, pre-pandemic childcare gaps within couples led children (and partners) to rely more heavily on mothers, even when both partners were home full-time. We discuss the implications for research on gender inequalities in paid work and parenting and for policy efforts to address gender inequalities at home.