This study evaluated the measurement invariance of the Mental Health Continuum-Short Form (MHC-SF), a 14-item self-report questionnaire for measuring emotional, social, and psychological well-being. The study draws on data of a representative panel (Longitudinal Internet Studies for the Social Sciences of CentERdata). 1,932 Dutch adults filled out the MHC-SF at four timepoints over 9 months. We used item response theory analyses with two-parameter models to examine differential item functioning across demographics, health indicators, and timepoints. The results indicated differences in the performance of one item (social well-being) for educational level, one item (social well-being) for sex, and two items (psychological well-being) for age. The MHC-SF is highly reliable over time, as there was no differential item functioning across the four timepoints. Furthermore, the means and reliabilities of the subscales were consistent over time. The MHC-SF is a reliable and valid instrument to measure positive aspects of mental health.