Several studies have indicated that dispositional mindfulness (DM) predicts better mental health in adolescents. The current study expands previous research by examining the reciprocal longitudinal associations between DM facets and psychological problems. In addition, the potential mediating role of self-concept (SC) dimensions is examined. A sample of 832 adolescents aged between 11 and 18 completed measures of DM, SC, and internalizing and externalizing problems in two waves six months apart. DM did not predict changes in psychological problems. However, in general, psychological problems predicted lower DM, some facets of DM predicted an increase in SC dimensions, and SC predicted higher scores on DM and fewer externalizing problems. In addition, acting with awareness mediated the relationship between externalizing problems and two SC dimensions. Findings highlight the beneficial role of having a positive SC for some dimensions of DM, and vice versa.