Background: While people with intellectual disabilities tend to refrain from physical activity, outdoor physical activity programs increase motivation to engage in physical activity. Method: Eighty adults with intellectual disabilities participated in a 12-month outdoor physical activity program. Attendance was used to assess feasibility. Aerobic capacity (6-min walk test), lower extremity endurance (30-s chair stand), and mobility (timed up and go) were assessed at three-time points: before, during, and after the program. Six interviews were also conducted with six staff members and participants. Results: The physical activity program was feasible, with all six groups completing the year-long activity. The six-minute walk and timed-up-and-go tests improved significantly. The qualitative analysis indicates the program's strengths (instructors' qualities and programs' social component) and weaknesses (dependency on weather and bureaucracy). Conclusion: Among adults with intellectual disabilities, a long-term outdoor physical activity program is feasible as a means for increasing aerobic capacity and improving mobility ability.