ObjectivesModerate mental health problems are highly prevalent and increasing in Swedish schoolchildren, elevating risk for future mental and somatic disability. The aim of this study was to determine whether an 8-week mindfulness-based intervention, Training for Mindfulness and Resilience (TMR), mitigates mental health symptoms and increases resilience during a 2-year follow-up.MethodsSchoolchildren (aged 9–14 years) reporting moderate mental health problems were randomized into either TMR intervention group (N = 22) or control group, receiving best current practice (N = 12). We used validated questionnaires to measure anxiety, depression, anger, disruptive behavior, self-concept, resilience, stress, and mindfulness before treatment with either TMR or control, as well as at 6 months, 1 year, and 2 years follow-up.ResultsWe found a statistically significant effect of TMR intervention vs control, on resilience, anxiety, anger, and disruptive behavior. Compared to baseline, TMR increased the level of resilience at 6 months (p < .001); anxiety at 1 (p < = .033) and 2 years (p = .04); anger at 6 months (p = .004) and 2 years (p = .039); disruptive behavior at 6 months (p = .006). In the control group, a decrease in resilience between 6 months and 2 years (p = .05) was observed. No other significant effects were found in the control group.ConclusionsThis study suggested that TMR improved mental health in schoolchildren with effects on anxiety and anger lasting for 2 years, and on resilience and disruptive behavior lasting for 6 months.Trial Registration NumberNCT04806542, date of registration 18th of March 2021, retrospectively registered.