Purpose: This study investigated the combined effects of mirror therapy involving power and strength exercises of the upper extremities in children with Unilateral Spastic Cerebral Palsy (USCP). Methods: Thirty children were included in either an experimental group or a control group. All participants were evaluated with the Quality of Upper Extremity Skill Test (QUEST), Canadian Occupational Performance Measure (COPM), and a handheld dynamometer to measure isometric muscle strength. Results: Compared to the control group, greater improvement was found in dissociated movements (p < .001, d = 1.82), grasp (p < .001, d = 1.38), weight bearing (p = .006, d = 0.91), and total scores (p = .001, d = 1.16) of QUEST; performance (p < .001, d = 2.9), satisfaction (p < .001, d = 1.91), and total scores (p < .001, d = 2.87) of COPM; and isometric muscle strength of the biceps brachii (p < .001, d = 1.27) and triceps brachii (p = .002, d = 2.22) of the affected upper limbs in the experimental group. Conclusions: Mirror therapy combined with power and strength exercises is a promising intervention approach to improve activity performance and upper-limb function in children with USCP.