The development of measures of children and adolescents’ subjective well-being is crucial to the conceptualization and evaluation of positive mental health. The aim of this study was to investigate the factorial structure and invariance of the Mental Health Continuum-Short Form (MHC-SF—adolescents) across children and youths. Participants were from two different samples: 208 elementary school children (sample 1) and 216 middle school youths (sample 2). Results confirmed the three-dimensional structure of subjective well-being in both samples. The three sub-scales of the MHCSF yielded high internal consistency and results from the HTMT85 indicated discriminant validity. Measurement invariance testing across three different age groups (7–8, 9–10 and 11–14 years) confirmed the full metric and approximate scalar invariance of the MHC-SF. Full scalar invariance was achieved across gender. The study also compared the latent means for mental well-being in the three age groups, and found that the younger groups showed significantly higher levels of wellbeing. The present research study strongly suggests that the MHC-SF (adolescents) is an appropriate instrument to measure the positive mental health and well-being of children and pre-adolescents as multidimensional construct.