Purpose: This study examined the extent to which irrational performance beliefs and intolerance of uncertainty co-occur in relation to mental well-being among a sample of athletes and coaches (N = 94, M age = 31.99, SD = 12.81) upon their return to sport following COVID-19 disruptions. Methods and Results: Despite the parity in views, independent samples t-test results identified three significant differences in the tested variables between athletes and coaches, which suggested that athletes are more likely to entertain depreciative thoughts about performances and react more aversively to uncertainty, whereas coaches reported a better mental well-being state. Pearson correlation coefficient analysis confirmed a significant positive relationship between composite irrational performance beliefs and intolerance of uncertainty scores, with both these variables being inversely related to mental well-being. Results from a simple atemporal mediation analysis using the PROCESS macro verified that intolerance of uncertainty fully mediated the adverse effect irrational beliefs exert on mental well-being. Conclusion: Sports psychology practitioners within the framework of REBT are advised to explore their orientation of modifying irrational beliefs aligned to clients’ perceptions and tolerance of uncertainty in sport through the inclusion of IU-specific awareness and behavioral experiments.