Mirror exposure (ME) is an effective technique to improve body image. However, evidence on the underlying mechanisms and the optimal verbalization instruction during ME is lacking. Therefore, this experimental study analyzed mechanisms of ME and therapeutic outcomes by comparing positive (PV) and negative (NV) full-body verbalization. N = 73 healthy females were randomized to a PV or an NV condition. PV participants verbalized positively while NV participants verbalized negatively about their whole body. Each participant underwent three standardized ME sessions. Before and after each ME session, positive affect, negative affect and body satisfaction were assessed. Before the first and after the third ME, participants completed questionnaires on cognitive-affective and behavioral aspects of body image, eating pathology and self-esteem. Regarding within-ME changes, the results indicate that positive affect and body satisfaction decreased while negative affect increased in the NV group but not in the PV group. In contrast, regarding between-ME changes, decreased negative affect as well as positive affect and increased body satisfaction were observed in both groups. However, eating pathology remained stable, whereas body-checking behavior increased and the PV condition was followed by higher levels of self-esteem compared to the NV condition. These findings suggest that both PV and NV improve negative affect and body satisfaction between-ME, and thus seem to be effective ME instructions. Given that NV led to increased negative affect within-ME and did not influence self-esteem, PV might represent the favorable instruction during ME for body-satisfied women.