This study examined the role of degree of adherence in a mindfulness-based intervention on mindfulness, flow, sport anxiety, and sport-related pessimistic attributions in athletes. Twelve athletes participated in an 8-week mindfulness intervention which incorporated a mindfulness focus on movement training component. Participants completed baseline and posttest measures of mindfulness, flow, sport anxiety, and sport-related pessimistic attributions, and they filled out daily mindfulness-training logbooks documenting their frequency and duration of mindfulness practice. Participants were identified as either high adherence or low adherence with mindfulness-training based on a composite score of logbook practice records and workshop attendance. Athletes high in adherence, operationalized as following recommended practice of mindfulness exercises, showed significantly greater increases in mindfulness and aspects of flow, and significantly greater decreases in pessimism and anxiety than low adherence athletes. Greater increases in mindfulness from baseline to posttest were associated with greater increases in flow and greater decreases in pessimism. Increases in flow were associated with decreases in somatic anxiety and pessimism.