Background: There is little previous research examining whether measures of working memory are related to educational achievement in children with intellectual disabilities (ID). Methods: A battery of working memory and achievement measures was administered to 11- to 12-year-old children with ID; younger typically developing children of comparable mental age were also assessed. Results: The working memory measures that assessed phonological short-term memory (PSTM) accounted for the most variance in reading and spelling in children with ID, whereas the working memory measures that assessed central executive-loaded working memory (CELWM) accounted for the most variance in number skills. These relationships were broadly similar among typically developing children. Conclusions: Compensatory strategies for weak PSTM may help to improve reading and spelling skills in children with ID, whereas reducing CELWM loads may be more helpful in aiding their number skills.