Although school engagement is crucial to child development, research on children with intellectual disabilities in mainstream schools is scant. This sequential, explanatory mixed-methods study examined the ways in and extent to which children with intellectual disabilities participate in mainstream school activities, as well the personal and environmental factors that affect their participation. A total of 101 general teachers provided quantitative data, while eight children with intellectual impairments and their teachers and peers provided qualitative data. In the quantitative survey, no significant differences were observed between children with and those without intellectual disabilities regarding school absence, but a low similarity existed in their extent of participation. Children with intellectual disabilities engaged most frequently in life-skills and after-school activities and least frequently in social and volunteer activities. School participation was affected by the degree of disability and environmental variables. We derived two themes from qualitative research: (1) school participation of children with intellectual disabilities; and (2) factors associated with school participation of children with intellectual disabilities. The results suggest strategies that may promote the participation of children with intellectual disabilities in mainstream schools.