We investigate the relationship between preschoolers' self-regulation and their mathematical competence and its development over the first two years of primary school using data from the German National Educational Panel Study (NEPS). Our results imply a positive association between self-regulation and mathematical competence levels, even when holding basic cognitive abilities constant. Self-regulation is, however, generally not related to competence development over the first two years of primary school. Children with low initial mathematical competence and, to some extent, children with migration background benefit from self-regulation at the transition to primary school but not between grades 1 and 2.