Mental fatigue leads to declines in performance of tasks such as cycling time trial performance and skill-based outcomes such as soccer shot accuracy. The neuromuscular mechanisms leading to these declines are not well understood. Although force variability has been shown to increase under dual-task conditions, it is not known if these results extend to conditions of mental fatigue. PURPOSE: The purpose of this study was to assess the impact of mental fatigue on variability in motor output in healthy, young individuals. Specifically, we sought to determine if a task that induces mental fatigue has effects on force and motor unit firing variability. METHODS: Nineteen participants (10 female, 9 male) performed 10-s isometric contractions at 20 and 50% maximum voluntary contraction (MVC) before, during, and after completing 20 min of the psychomotor vigilance task (PVT). The PVT is a sustained attention task that induces mental fatigue, as indicated by increases in reaction time (RT) to visual stimuli. Force and indwelling motor unit (MU) firings were measured prior to and immediately following performance of the PVT (single task), and within the first and final minutes of PVT performance (dual task). Subjective ratings of fatigue were also obtained using a 10-point Likert scale before and after the PVT. RESULTS: Reaction time increased by 14% from the beginning of the PVT (276.14 ± 31.54 ms) to the end (314.15 ± 37.74 ms, p<0.001). Subjects also subjectively reported greater levels of fatigue following the PVT (4.95 ± 1.84) compared to before (3.00 ± 1.20, p<0.001), indicating successful induction of mental fatigue. For the coefficient of variation (CV) of force there was no significant main effect of time (p=0.14) or contraction intensity (p=0.33), and no significant interaction (p=0.51). For the CV of the MU interspike interval there was a main effect of contraction intensity with a greater CV of the interspike interval during the 50% MVC (19.51 ± 3.84%) than the 20% MVC (14.87 ± 3.59%, p<0.001) contractions. However, there was no significant main effect of time (p=0.83) and no significant interaction (p=0.23). CONCLUSION: Inducing mental fatigue did not lead to changes in the variability of force production or motor unit firing during isometric contractions at 20 and 50% MVC as a single-task or during a concurrent cognitive task.