Individuals with autism spectrum disorder (ASD) are at higher risk for developing common chronic diseases. Engagement in physical activity (PA) can prevent health issues; however, people with ASD are known to engage in lower levels of PA in comparison to their peers. This study evaluated the effect of a long-term, 12-month PA intervention on the fitness and quality of life of adults with ASD. A quantitative approach was implemented to assess participants' fitness, functional ability, quality of life, and participation in a range of PA classes at three different time points. Qualitative data were collected via in-depth, semi-structured interviews with three participants with ASD and three staff members. A total of 34 adults with ASD (mean age 39.76 + 7.27) participated in the quantitative part of the study. Approximately 53% of the participants exhibited perseverance and conducted adequate PA each month. Significant improvements were found in one fitness component and two quality-of-life components. Factors revealed for the program's success were the individuals' free choice of the PA classes and supporting people and a budget that tailored the project. Policymakers who plan health promotion programs for adults with ASD should consider long-term PA programs, with freedom of choice among PA modalities and schedules.