This study examined the effects of group art therapy on self-concept and peer relationships among adolescents using a mixed-method approach. Twelve recruited adolescents from a high school in Northern Taiwan attended weekly 2-h group art therapy sessions for 8 weeks. Through a discovery-oriented approach incorporating focus group interviews and individual interviews, qualitative results revealed that the participants started getting to know themselves better and were able to explore their inner selves while noticing their own emotions. Their self-concept also became more positive along with reduced negative self-concept, increased positive self-concept, and clearer visions towards future goals. Furthermore, the therapy modified adolescents' negative peer relationships by reducing their social anxiety and adjusting their complicated experiences. Participants were found to experience trustable, allied, and compassionate positive peer relationships while having reduced loneliness and improved social skills at the same time. Besides, the participants answered self-concept and peer relationship scales at pre- and post-interventions as well as a 12-month follow-up thereafter. Friedman test on the longitudinal data showed participants reporting consistently higher self-concept and peer relationship at post-intervention and 12-month follow-up than at the pre-intervention stage. Taken together, the findings duly support art therapy in boosting adolescents' self-concept and peer relationships.