The relationships between perceived fatigue and changes in sustained attention performance during early stages of working on cognitively demanding tasks remain poorly understood. In addition, concerns have been raised that self-ratings of fatigue may be biased by socially desirable response tendencies, potentially confounding the relationship between perceived fatigue and attention performance. In this study, we assessed perceived fatigue briefly before tracking changes in concentration performance, processing speed, and error rates among N = 110 tenth graders, while they completed the d2-R (...) test of sustained attention. By statistically controlling for social desirability, we examined relationships between perceived fatigue and the initial levels and slopes of three latent growth-curves capturing changes in the d2-R test’s performance measures. Individuals with higher fatigue exhibited lower concentration performance, a weaker decline in processing speed, and a higher error rate over the course of testing. Post hoc power analyses supported the robustness of our results. Implications for mental fatigue research are discussed. (shrink)