Background: The theoretical models of Beck and Young predict that Early Maladaptive Schemas (EMSs) are linked to the development and maintenance of mental health problems including depression. The stability of EMSs in daily life are ill-understood despite being a fundamental aspect of cognitive theories. In the current study, we aimed to improve the understanding of EMSs by repeatedly assessing them in daily life under changing contextual triggers and examining the theoretically-grounded associations with mood and rumination. Methods: Using factor analysis, we developed a 16-item Ecological Momentary Assessment (EMA) version of the Dutch Young Schema Questionnaire short form (Klynstra et al., 2008). EMSs were assessed six times a day for five days in 90 unselected participants (71% female, Mage = 25.52). Results: The new EMA-based EMSs questionnaire captured EMSs fluctuations, with 51% of the variance attributed to within-person variations. We assessed the contemporaneous within-person associations between schema activation and negative affect and rumination as well as the impact of a triggering event on schema activation. Stronger EMSs activation was associated with more negative mood and rumination, as well as the occurrence of a recent triggering event. Conclusions: The findings align with the cognitive models of Beck and Young extending them with daily life data. The results indicate that EMSs have both state and trait-like characteristics, and fluctuations in daily life can be assessed.