Although a growing body of research has demonstrated the benefits of mindfulness for depression and anxiety, the mechanisms by which mindfulness reduces emotional distress are unclear. At least two mechanisms have been proposed: reduced negative cognitive bias and stress reduction. Although both mechanisms have received initial support, these proposed mechanisms have not been examined concurrently. The present studies examined the extent to which less negative cognitive bias and less perceived stress uniquely accounted for the association between mindfulness and emotional distress. In two studies, participants completed measures of trait mindfulness, perceived stress, negative cognitive bias, depression, and anxiety. Across both studies, results from parallel multiple mediation models indicated that both negative cognitive bias and perceived stress accounted for unique variance in the mindfulness–emotional distress association. That is, greater mindfulness was related to less negative cognitive bias and less perceived stress, which in turn were associated with less emotional distress. The results suggest that both stress reduction and negative cognitive bias may be mechanisms by which mindfulness confers benefits to psychological well-being.