OBJECTIVES: Mindfulness-based interventions (MBIs) have been shown to be beneficial for young adults with adverse childhood experiences (ACE), but the temporal course of the therapeutic effects is still not well understood. This study aimed to investigate MBI-induced weekly changes and long-term effects in trait mindfulness and psychological symptoms. METHOD: This study analyzed longitudinal data from a mechanistic clinical trial in which young adults (aged 21-35) with ACE were randomized to an 8-week MBI or an active control condition of Stress Management Education (SME), with 21 and 19 completers respectively. Mindful Attention Awareness Scale (MAAS), Perceived Stress Scale (PSS), and Kellner's Symptom Questionnaire (KSQ) were administered before, after, and weekly during the 8-week interventions, and at 6-, 12-, and 18-months follow-up. Data analyses were conducted with linear mixed effects models, Granger causality and Dynamic Structural Equation Modeling (DSEM). RESULTS: Group by time interaction effects were observed with PSS, MAAS, KSQ-depression, KSQ-hostility, and KSQ-anxiety with small effect sizes. Only in the MBI group, increases of MAAS scores predicted reductions of PSS, KSQ-somatization, and KSQ-hostility scores. MAAS score reached significant increase at the end of MBI, and most MBI-induced symptom reduction lasted 12 months without refresher courses. CONCLUSIONS: MBI demonstrated unique effects of trait mindfulness improvement leading to psychological symptom reductions. At least 8 weeks of MBI program duration and refreshment at 12 months may be necessary for ACE survivors. PREREGISTRATION: This study is not preregistered.