Objective: Through intensive experience sampling, we studied the practice and development of mindfulness as a dynamic process in time and context. We focused on role(s) and salutary function(s) of mindfulness and decentering for emotional experience over the course of mindfulness practice and development. Method: Eighty-two meditation-naive adults from the general community, 52% women, Mage (SD) = 25.05 (3.26) years, participated in a 1-month, 6-session, Mahasi-based mindfulness-training intervention (Mahasi, 1978). We collected 52 digital experience samples of mindfulness, decentering, and emotional experience, in the context of daily living and meditative states, over the course of the program. Results: Data were analyzed via time-varying effects models (TVEMs) and mixed-linear models (MLMs) within a single-subject, multiple-baseline experimental design. First, over the course of the intervention, participants grew more mindful and decentered in daily living and meditative states. Second, the association between mindfulness and decentering was significant in daily living, although the magnitude of this association was stronger in meditative states. Third, we observed the same contextualized pattern of relations between mindfulness and emotional valence (happy > sad) as well as arousal (calm > nervous). Finally, whereas decentering mediated the effect of mindfulness on reduced emotional arousal in meditative states, it did not similarly mediate the effect of mindfulness on positive emotional valence. Conclusions: The present findings illustrate the insights that may be gained about mindfulness mechanisms broadly and decentering specifically through the study of mindfulness as a dynamic, contextualized developmental process over time.