Purpose Mental practice (MP) is a method of rehabilitating upper extremity function on the affected side of the body post-stroke, with the aim of improving motor task performance through the sustained repetition of motor imagery (MI). However, most studies thus far have investigated MP for post-stroke paralytic upper limb function in patients in the chronic phase. Therefore, it is necessary to obtain evidence regarding whether MP is an effective intervention modality in the acute phase of stroke. In the present study, we examined the effects of an intervention combining mirror therapy and MP initiated during the acute phase of cerebral infarction. Participant and Methods A female patient >80 years of age with a cerebral infarction was studied. Prior to cerebral infarction, the patient was independent in her activities of daily living. Results As a result of MP, sufficient improvement was observed in the upper extremity function on the paralyzed side, as assessed using the Fugl–Meyer Assessment (FMA) and Motor Activity Log (MAL). Conclusion In patients with MP initiated during the acute stroke phase, a combination of mirror therapy and action observation to enable vivid MI may elicit a more significant intervention effect.