Background: Mothers of children with intellectual disabilities are more likely to show elevated psychological distress, with child behavioural and emotional problems being a significant risk factor. Family Resilience Theory suggests that family relationships are crucial in influencing adaptation to stressors. We investigated whether family functioning mediates or moderates the relationship between child behavioural and emotional problems and subsequent maternal psychological distress. Method: We conducted mediation and moderation analyses on data from 324 mothers of children with intellectual disabilities in a United Kingdom (UK) prospective longitudinal cohort study. Results: Family functioning had a small mediating effect on the relationship between child behavioural and emotional problems and subsequent maternal psychological distress. Family functioning did not moderate the relationship between child behavioural and emotional problems and later maternal psychological distress. Conclusions: The mechanism of the effect of child behavioural and emotional problems on maternal mental health may at least partially involve changes in family functioning.