Several studies suggest that when subjects mentally rehearse or execute a familiar action, they engage similar neural and cognitive operations. Here, we examined whether muscle fatigue could influence mental movements. Participants mentally and actually performed a sequence of vertical arm movements (rotation around the shoulder joint) before and after a fatiguing exercise involving the right arm. We found similar durations for actual and mental movements before fatigue, but significant temporal discrepancies after fatigue. Specifically, mental simulation was accelerated immediately after fatigue, while the opposite was observed for actual execution. Furthermore, actual movements showed faster adaptation (i.e., return to prefatigue values) than mental movements. The EMG analysis showed that postfatigue participants programmed larger, compared to prefatigue, neural drives. Therefore, immediately after fatigue, the forward model received dramatically greater efferent copies and predicted faster, compared to prefatigue, arm movements. During actual movements, the discrepancy between estimated (forward model output) and actual state (sensory feedback) of the arm guided motor adaptation; i.e., durations returned rapidly to prefatigue values. Since during mental movements there is no sensory information and state estimation derives from the forward model alone, mental durations remained faster after fatigue and their adaptation was longer than those of actual movements. This effect was specific to the fatigued arm because actual and mental movements of the left nonfatigued arm were unaffected. The current results underline the interdependence of motor and cognitive states and suggest that mental actions integrate the current state of the motor system.