Researchers have begun to examine the psychological toll of the ongoing global COVID-19 pandemic. Data are now emerging indicating that there may be long-term adverse effects of the pandemic on new mothers and on children born during this period. In a longitudinal study of maternal mental health and child emotional development during the pandemic, we conducted online assessments of a cohort of women at two time points: when they were pregnant at the beginning of the surge of the pandemic in the United States (baseline, N = 725), and approximately 1 year postpartum (follow-up, N = 296), examining prenatal and postnatal maternal mental health, prenatal pandemic-related stress, and infant temperament. Pandemic-related stress at baseline was associated with concurrent depressive symptoms and infant negative affect at follow-up. Baseline maternal depressive symptoms were associated with follow-up depressive symptoms, which in turn were also associated with infant negative affect. Pandemic-related stress during pregnancy may have enduring effects on infant temperament. These findings have important implications for our understanding of the emotional development of children who were in utero during the COVID-19 pandemic.