Maternal depression may affect children through various mechanisms, including the stressful contexts of the children’s lives and maternal maladaptive affection, behaviors, and cognition, which affect the psychopathological development of children and contribute to disorders. The aims of the present study were to explore how family functioning mediates the relationship between maternal depression and the positive and negative emotions of adolescents. A total of 430 paired questionnaires were collected from junior high school students and their mothers. The mothers completed the Center for Epidemiologic Studies Depression Scale (CESD), and the adolescents completed the CESD, Mood and Anxiety Symptom Questionnaire, and Family Function Scale. The results revealed significant correlations among maternal depression, family functioning, and positive and negative emotions in the adolescents. The family functioning subscales of conflict and emotional expression partially mediated the relationship between maternal depression and the positive and negative emotions of the adolescents. Furthermore, the family functioning subscales of cohesion, problem solving, and responsibility fully mediated the relationship between maternal depression and positive moods in the adolescents. Analysis of the results for maternal depression, family functioning, and the emotional adjustment of the adolescents indicated that family functioning is the mediator between maternal depression and the positive and negative emotions of the adolescents. The current results suggest that adequate family functioning can prevent depression being transmitted from mothers to their adolescent children.