Abstract Background Mental illnesses are the leading cause of disability in young people, and lifestyle interventions in young people at risk of mental illness remain a priority. Opportunities to improve nutrition and physical activity among young people through youth mental health services remain unclear. This study aimed to determine the knowledge and behaviors towards nutrition and physical activity, the barriers and enablers to improving behaviors, and the preferred providers and sources of information for nutrition and physical activity among a sample of young people attending a youth mental health service. Methods A mixed-method study was conducted in regional Tasmania, Australia in a sample of young people (15–25 years) attending a youth mental health service (headspace). A quantitative survey (n = 48) determined young people’s nutrition and physical activity knowledge, behaviors, barriers and enablers to achieving recommendations, and their preferred providers and sources of information. Structured interviews and a focus group further explored these concepts (n = 8), including the role of the mental health service as a provider of this support. Results The majority of participants did not meet national recommendations for nutrition and physical activity, despite possessing a high level of knowledge regarding their importance for mental health. Improving mental health was a common enabling factor for participants choosing to alter diet and physical activity habits, but also the leading barrier for participating in physical activity. Young people wanted to receive information from reputable health providers, ideally through social media sources. headspace was seen as an important potential provider of this information. Conclusions Our results indicate that there is a clear need to improve diet and physical activity habits to enhance mental and physical health outcomes in this at-risk group, and youth mental health services could provide further interventions to support their clients. Specialized staff (e.g. dietitians and exercise physiologists) may provide additional benefits alongside existing mental health care support.