Uncertainty represents a robust threat that can amplify aversive experiences and exaggerate negative expectations about uncertain future outcomes. Mindfulness – an open and receptive attention to present moment experiences -- has been shown to facilitate adaptive regulation when faced with a variety of distinct emotional threats. Reduced experiential avoidance and equanimity in the face of unpleasant emotional experiences have been theorized as central to these emotional regulatory benefits. The present study explored whether dispositional mindfulness would promote adaptive responses to uncertainty during the anticipation of, and after exposure to emotional stimuli, as indicated by self-reports and neural (event-related potential) markers of anticipation and appraisal. Participants were exposed to stimulus cues that informed them about the valence of a subsequent emotional picture as neutral, aversive, or uncertain. Consistent with past research, uncertainty during the anticipation of an emotional stimulus amplified unpleasant stimulus appraisals, and participants demonstrated biased expectations to associate uncertainty with aversiveness. Dispositional mindfulness was associated with lower expectations for unpleasant stimuli, and was found to amplify the effect of uncertainty on a cortical marker of stimulus appraisal called the late positive potential (LPP). Traits that contrasted with mindfulness predicted opposite patterns of association with these measures. However, these findings were directly the opposite of findings from past research. A theoretically defensible explanation is discussed for these findings and suggestions were made for future research on the role of mindfulness on ERP variability. The results from the present study contribute to a growing body of evidence that suggests that uncertainty during the anticipation of potentially negative future outcomes can exert a potent downstream influence on emotional anticipation and appraisal processes. Further research is needed to clarify the role of dispositional mindfulness during emotional stimulus anticipation and appraisal following uncertainty.