Background: Mothers with lifelong care responsibilities might involuntarily be non-employed or work part-time, both of which are defined as "underemployment." This study aimed to investigate who these underemployed mothers are and what are the factors associated with such employment hardship when having a child with intellectual disability (ID). Method: An interview survey was conducted in 2011 in two local authorities of Taiwan on 876 working-age mothers with a child with intellectual disability; 514 of them were working part-time/non-employed and chosen as participants of this study. Results: The mothers with a younger child with intellectual disability, a higher level of education, a lower level of family income and more family members with disabilities were more likely to be underemployed compared with the mothers who were voluntarily working part-time/non-employed. Conclusions: The underemployed mothers were more likely to have financial difficulty and heavy caregiving loads; their employment hardship should be of concern for policymakers.