Mindfulness-based interventions are efficacious in reducing depression and anxiety symptoms, but mechanisms underlying these benefits have not been fully elucidated. Based on cognitive theories of depression and anxiety, a reduction in negatively biased cognition may be one mechanism. We reviewed the empirical literature on mindfulness and negatively biased cognition. Thirty-seven studies met inclusion/exclusion criteria. The evidence generally indicated that mindfulness was inversely associated with negatively biased cognition, across various measures (i.e., self-reported negatively biased cognitive styles and cognitive responses to tasks involving valenced stimuli), study designs (i.e., correlational, quasi-experimental, and experimental), and samples (i.e., unselected and clinical). However, the relation was mixed for cognitive reactivity to negative emotions in recurrent depression and null for indices of quick behavioral control. Correlational evidence in unselected samples also supported an indirect relation between mindfulness and fewer depression and anxiety symptoms through less negatively biased cognition. Mindfulness may be useful for addressing this intended mechanism of traditional cognitive-behavioral therapy. Future research should more directly test the role of negatively biased cognition in protective and therapeutic effects of mindfulness.