Rates of mental health challenges among children and youth are on the rise. Physical activity has been identified as a promising intervention to improve mental health outcomes for youth. This study aimed to investigate how mental health clinicians perceive and utilize physical activity as a mental health intervention for children and adolescents. Seventy-four Ontario mental health clinicians (psychologists, psychotherapists, and social workers) were surveyed about their perspectives and practices related to physical activity as part of mental health care for children and adolescents using a mixed-methods approach. Survey respondents were also asked about barriers to including physical activity in care. Although 100% of clinicians agreed that physical activity was beneficial to their own and to their clients' mental health, only 61% reported prescribing physical activity to their clients. Barriers to prescribing physical activity as a treatment option included lack of training, time, and resources. Clinicians who were more physically active themselves were more likely to view physical activity as beneficial for mental health and were less likely to view time as a barrier to discussing and prescribing physical activity in their practice. Many clinicians expressed a need for more training, knowledge, and resources. The results of this study suggest that with improved access to training and resources, clinicians will be well-positioned to leverage the benefits of physical activity for mental health in their practice.