Previous studies reported that a substantial number of students with autism spectrum disorder (ASD) encounter challenges in mathematics as they advance through their education. The present study evaluates the mathematical competence of 17 students diagnosed with ASD who were recruited from first through fourth grades in 12 different mainstream schools. The results were compared with those from other 17 students without ASD, matched one-to-one by sex and school (grade and classroom). In general, the students with ASD in this study exhibited greater difficulties in mathematics than their peers without ASD. These difficulties were particularly noticeable in fourth graders, although they could be observed earlier in informal skills. Many students with ASD in the sample struggled with informal calculation and concepts, as well as with all formal categories considered, particularly calculation and concepts. These findings may be relevant for early teaching interventions and for the development of future mathematical learning supports for students with ASD.