Based on the self-system model of motivational development, we examined the mediating role of school engagement, including cognitive engagement and behavioral engagement, in accounting for the relation between mastery goal orientations and subjective well-being in school among elementary school students. A total of 974 Chinese children (440 females, 534 males; M[subscript age] = 9.52, SD = 0.95) completed multiple self-report questionnaires at four time points, 6 months apart, across a 2-year period. Structural equation modeling was used to examine the hypotheses, controlling for the initial levels of mediating and dependent variables, and demographic variables (i.e., gender, age, and parental education levels). Results indicated that (a) mastery goal orientations at Time 1 positively related to subjective well-being in school at Time 4; (b) cognitive engagement at Time 2 mediated the relation between mastery goal orientations at Time 1 and subjective well-being in school at Time 4; and (c) mastery goal orientations at Time 1 displayed indirect effects on subjective well-being in school at Time 4 successively via the chain mediation of cognitive engagement at Time 2 and behavioral engagement at Time 3. These findings suggested that, in order to promote positive subjective well-being in elementary school students, school professionals and parents should simultaneously encourage students to cultivate mastery goal orientations and high levels of cognitive and behavioral school engagement.