Background: Many previous studies have reported that mothers of children with intellectual disabilities (IDs) are more likely to show signs of psychological distress and have lower well-being than mothers of "typically developing" children. Our aim was to estimate the extent to which these differences may be accounted for by between-group differences in socio-economic position. Methods: This study involved secondary analysis of happiness, self-esteem and self-efficacy variables in a nationally representative sample of 6954 British mothers with dependent children under the age of 17 years, 514 of whom were supporting a child with an ID. Results: Mothers of children with IDs reported lower levels of happiness, self-esteem and self-efficacy than mothers of children without IDs. Statistically controlling for differences in socio-economic position, household composition and maternal characteristics fully accounted for the between-group differences in maternal happiness, and accounted for over 50 of the elevated risk for poorer self-esteem and self-efficacy. Conclusions: A socially and statistically significant proportion of the increased risk of poorer well-being among mothers of children with IDs may be attributed to their increased risk of socio-economic disadvantage.