Background: Physical inactivity is a leading risk factor for mortality. Adults with intellectual disability are extremely inactive, but less is known about physical activity levels in children and youth with intellectual disability. This paper examines the participation by adolescents and young adults with and without mild to moderate intellectual disability in sport/exercise. Methods: Secondary analysis was undertaken of Next Steps, an annual panel study that followed a cohort from early adolescence into adulthood. Participants with mild to moderate intellectual disability were identified through data linkage with educational records. Results: Sport/exercise participation rates were consistently lower for adolescents and young people with mild to moderate intellectual disability than for their peers without intellectual disability. Matching participants on between-group differences in exposure to extraneous risk factors did not impact on these between-group differences in participation in sport/exercise. Conclusion: The results support limited existing evidence regarding the low level of participation of children and young people with intellectual disability in sport/exercise compared with their peers. Future work on promoting sport/exercise and physical activity in children and young people with intellectual disability may play a role in helping to reduce the health inequalities experienced by people with intellectual disability.