Two studies explored the relationship between emotions, perceived mental effort and concentration disruption in adult sport performers. In Study 1, semi-professional association football players completed questionnaire measures before and after a competitive match. In Study 2, student athletes completed questionnaire measures for two performance scenarios: one in which they were performing above their normal level and one in which they were performing below their normal level. Findings demonstrated that cognitive trait anxiety was associated with greater disruptions in concentration but was unrelated to mental effort. For state measures, athletes reported greater levels of concentration disruption when experiencing high levels of anxiety or high levels of happiness, and fewer disruptions in concentration when experiencing high levels of excitement. Findings also showed that excitement was associated with low levels of mental effort during good performances and high levels of mental effort during poor performances; anxiety and happiness were associated with high levels of mental effort during good performances and low levels of mental effort during poor performances. Taken together, these studies point towards potential benefits accompanying high levels of excitement and potential disadvantages accompanying high levels of anxiety and happiness.