Stroke is one of the leading causes of death and disability worldwide and poses a tremendous socioeconomic burden upon individuals, countries and healthcare systems. It causes debilitating symptoms and thus interferes with many aspects of the patient’s life, including physical functioning, cognition, emotional status, activities of daily living, social reintegration and quality of life. Post-stroke patients frequently experience functional motor disabilities of the upper limb, which restrict autonomy and self-efficacy and cause limitations in engagement with activities and social participation, as well as difficulties in performing important occupations. It is therefore not surprising that motor impairment or loss of motor function of the upper limb is one of the most devastating sequelae of stroke. On these grounds, achieving optimal functioning of the upper limb after stroke remains a fundamental goal of stroke rehabilitation. Mirror therapy (MT) represents one of the several rehabilitation techniques used for restoring the upper limb’s motor function after a stroke. However, conflicting results about the role of MT in the rehabilitation of the upper limb’s motor deficits have been reported in the literature. Accordingly, the aim of this narrative review is to summarize existing evidence regarding the effects of MT on the upper limb’s motor function in post-stroke patients and to further explore its role when applied in different phases of stroke.