Background: Adolescents and young adults with intellectual and developmental disabilities are at risk of obesity. Parents influence their diet and physical activity behaviours and therefore, can play important roles in weight management. The aims of this qualitative study were to explore parents' experiences assisting their son or daughter to participate in a weight management study. Methods: Interviews were completed at 6 months with 27 parents whose adolescent or young adult had completed the weight loss portion of an 18-month weight management study. Interviews were recorded, transcribed and thematic analysis performed. Results: Parents shared insights about how well program components worked with their family, and what strategies worked best to adopt healthier dietary choices and become more physically active. The importance of meeting regularly with someone outside the family to encourage healthier habits was stressed. Conclusions: Future weight management studies should involve parents and their adolescents to help tailor strategies and adapt intervention approaches.