Background: Despite an increase in international studies examining the experiences of parents with intellectual impairments and their children, few have utilised population-based data. This study investigated the behavioural and cognitive outcomes of 3-year-old US children of mothers with intellectual impairments compared with children of mothers without intellectual impairments. Methods: This study employed a secondary analysis of the Fragile Families Child and Wellbeing Study, a longitudinal birth cohort study in the US. Our analytic sample included mothers with intellectual impairments (n = 263) and a comparison group of mothers without intellectual impairments (n = 1298), as well as each sampled mother's focal child. When weighted, Fragile Families is representative of all births in US cities with populations over 200 000. Results: Children of mothers with intellectual impairments had poorer behavioural and cognitive outcomes in comparison to same-age children of mothers without intellectual impairments. Notably, however, children of mothers with intellectual impairments were not at increased risk of being aggressive unless their family income was below 200% of the federal poverty level. Further, families headed by mothers with intellectual impairments experienced multiple hardships related to socioeconomic factors, limited social supports and poor self-reported health. Conclusion: Appropriate policies and programmes must be developed and implemented to effectively support these families, such as increased financial benefits.