This study examined mathematics achievement in school-aged children with autism spectrum disorder (ASD), symptoms of attention-deficit/hyperactivity disorder (ADHD), or typical development (TD) over a 30-month period and the associations between cognitive and reading abilities with mathematics achievement in children with ASD. Seventy-seven children with ASD without intellectual disability (ASD-WoID), 39 children with ADHD, and 43 children with TD participated in this study. The results revealed that the ASD-WoID and ADHD samples displayed significant and comparable delays in problem solving and calculation abilities. Lower VIQ was related to lower math achievement across all subgroups. The ASD-WoID sample differed from comparison samples in terms of their pattern of mathematical achievement and the role of cognitive abilities in the development of mathematics competence.