Lower-body robotic exoskeletons can be used to reduce the energy demand of locomotion and increase the endurance of wearers. Understanding how motor fatigue affects walking performance may lead to better exoskeleton designs to support the changing physical capacity of an individual due to motor fatigue. The purpose of this study was to investigate the effects of motor fatigue on walking mechanics and energetics. Treadmill walking with progressively increased incline gradient was used to induce motor fatigue. Twenty healthy young participants walked on an instrumented treadmill at 1.25 m/s and 0° of incline for 5 min before (PRE) and after (POST) motor fatigue. We examined lower-limb joint mechanics, metabolic cost, and the efficiency of positive mechanical work (η⁺_work). Compared to PRE, participants had increased net metabolic power by ∼14% (p <​ 0.001) during POST. Participants also had increased total-limb positive mechanical power (Total P⁺_mech) by ∼4% during POST (p <​ 0.001), resulting in a reduced η⁺_work by ∼8% (p <​ 0.001). In addition, the positive mechanical work contribution of the lower-limb joints during POST was shifted from the ankle to the knee while the negative mechanical work contribution was shifted from the knee to the ankle (all p <​ 0.017). Although greater knee positive mechanical power was generated to compensate for the reduction in ankle positive power after motor fatigue, the disproportionate increase in metabolic cost resulted in a reduced walking efficiency. The findings of this study suggest that powering the ankle joint may help delay the onset of the lower-limb joint work redistribution observed during motor fatigue.