Aims: Supervised exercise provides numerous health benefits for people with type 2 diabetes mellitus (T2D); however, long-term adherence is poor. We aimed to identify (1) factors associated with total physical activity (PA) participation 10 months following supervised exercise and (2) barriers to and enablers of self-directed exercise. Methods: Fifty-seven adults with type 2 diabetes mellitus (age: 60.4 [8.6] y, 23% women, glycated hemoglobin [HbA1c]: 8.7% [1.9%], PA: 68.1 [142.3] min·wk−1) were allocated to low-volume combined aerobic and resistance high-intensity interval training (78 min·wk−1) or combined moderate intensity continuous training (210 min·wk−1). Participants completed 8 weeks of supervised training, then 10 months of self-directed exercise (12 mo total). Sociodemographic, medical, psychological, and health determinants of total PA at 12 months were identified using correlation and multiple regression analyses. Barriers and enablers were derived from individual participant interviews. Results: At 12 months, average weighted total PA was 209.4 (174.3) minutes per week. Only allocation to combined aerobic and resistance high-intensity interval training was an independent predictor of total PA at 12 months (β = 0.477, P = .027). Enablers of self-directed exercise included flexible exercise schedule and social support. Barriers were limited access to specialized equipment and competing time demands. Conclusions: This study highlights factors to consider when designing long-term exercise support for people with type 2 diabetes mellitus.