Background: Earlier research shows that nonemployed mothers of children with intellectual disability (ID) have lower wellbeing than employed mothers. This study explored why and to what extent these mothers did not participate in the labour market. Method: An in-depth interview was employed, and 18 working-age and nonemployed mothers in Taiwan who had an adult child with ID were interviewed in their homes between July 2009 and May 2010. Results: The mothers left the labour market at different stages of the family life cycle due to a lack of formal/informal support for the care needs of their young children, and the continuing intensive care needs of their child with ID. Mothers were officially nonemployed; however, to meet their family's financial needs, they were hidden workers in practice. Conclusions: Policies are required that support these hidden female workers, who are also lifelong carers, by offering financial support and affordable social services.