Cognitive theorists describe mindfulness as a form of attention-awareness in which thoughts can be observed in non-judging, de-centered, and non-attached ways. However, empirical research has not examined associations between mindfulness and responses to negative automatic thoughts, such as the ability to let go of negative cognition. In the first study reported in this article, measures of dispositional mindfulness were negatively correlated with negative thought frequency and perceptions of the ability to let go of negative thoughts in an unselected student sample. In the second study reported, these associations were replicated in a treatment-seeking student sample, where participation in a mindfulness meditation-based clinical intervention was shown to be associated with decreases in both frequency and perceptions of difficulty in letting-go of negative automatic thoughts. Theoretical and clinical implications are discussed.