This study investigated triathletes’ metacognitions and mindfulness traits (N = 232) measured prior to competition, and flow (N = 63), post competition. The primary aim was to investigate whether metacognitions (measured by the Metacognitions Questionnaire) would associate with mindfulness facets (measured by the Five Facet Mindfulness Questionnaire – Short Form), and metacognitions would also predict flow scores (measured by the Short Flow State Scale), over and above mindfulness facets. Regression analyses showed that metacognitions individually predicted mindfulness facets. A hierarchical regression showed that positive beliefs about worry negatively predicted flow, while a lack of cognitive confidence, beliefs about the need for thought control and acting with awareness positively predicted flow. These findings indicate that a) metacognitive beliefs are influential to cognitive predispositions, b) typically dysfunctional metacognitions may play a different role in competitive environments, and c) metacognitions may play a more important role in the occurrence of flow, than mindfulness.