This study investigated the developmental interdependence of Finnish school-beginners’ (N = 285) ability self-concept, intrinsic value, and performance in mathematics. More specifically, we examined: (i) whether and how children’s ability self-concept and intrinsic value in mathematics change over their first three years in school, (ii) how those changes are related to each other, (iii) how they predict later math performance, and (iv) whether there are gender differences in these trajectories. The results showed significant decrease over time in children’s ability self-concept and intrinsic value, but also significant individual differences in the trajectories. The high dependency between the levels and changes in children’s self-concept and intrinsic value led us to specify a factor-of-curves latent growth model, thus merging the trajectories of ability self-concept and intrinsic value into one common model (i.e., math motivation). The subsequent results showed prior math performance to predict change in children’s math motivation, meaning that higher initial competence was connected with less steep decrease in motivation. After controlling for the effects of first-grade math performance, both the level and change in math motivation predicted third-grade math performance and teacher-rated grades. That is, higher initial motivation and less steep decrease in it independently predicted better later math competence. Boys reported less steep decrease in math motivation than girls, despite no gender differences in initial math performance.