Trait mindfulness helps people handle distress and improves their satisfaction with life. The aim of the present paper is to examine whether trait mindfulness also promotes positive functioning (i.e., performance) in highly demanding situations, such as in elite sports. Mindfulness has been shown to improve athletes’ ability to perform well, i.e., to increase their performance-delivery. However, researchers are still speculating about the underlying mechanism. The present research examines whether trait mindfulness enhances the ability of elite athletes to trigger performance in demanding situations by generally reducing competition anxiety and diminishing its negative impact when it occurs. Participants were 133 elite athletes from 23 different sports. They completed measures of trait mindfulness, competition anxiety, and performance-delivery. Mediation, moderation, and moderated-mediation effects of mindfulness and competition anxiety on performance-delivery were tested. Our findings indicate that trait mindfulness is related to fewer performance worries and prevents the remaining worries from influencing athletes’ behavior, thereby helping them to perform better. Implications and directions for further research are discussed. Apart from benefits for psychological health, instructing people to become more mindful might be a promising approach to help them optimize their performance in demanding situations.