Although there are many benefits to regular engagement in physical activity, adults with intellectual and developmental disabilities often do not engage in or near the recommended amount of physical activity. Barriers, such as lack of perceived capability, accessible environments, transportation, social support, and or knowledgeable support staff, may limit participation in physical activity. The present study used qualitative methods to explore the experiences of adults with intellectual and developmental disabilities enrolled in a fitness program. We used field observations and photo-elicited semi-structured interviews to explore what capabilities, opportunities, and motivations facilitate or hinder engagement in fitness classes and their experiences in the program. We used the COM-B model to deductively interpret and analyze the data through thematic analysis. Major themes were identified around types of support and preferences for physical over sedentary activities. Instructor, client, and family support were identified as important in facilitating interest, engagement, and skill. Dependency on others for financial and transportation support was also reported as important for participants to access the fitness program. This study provides valuable insight into the interactions and experiences of adults with intellectual and developmental disabilities, including what keeps them engaged in a fitness program as it relates to capabilities, opportunities, and motivation.