Background: This study investigated whether reported neighborhood variables explained variance in time spent walking, exercising, and being sedentary, in addition to mental health and demographic variables among Ugandan outpatients with mental illness. Methods: Ninety-nine outpatients (78 men; 31.1 [8.6] y) of the Butabika National Referral Hospital in Uganda completed the Neighborhood Environment Walkability Scale for Africa, the Simple Physical Activity Questionnaire, and the Brief Symptoms Inventory-18. Multiple regression analyses were performed. Results: Seven percent of the variance in walking time was explained by the variance in anxiety/depression and an additional 13% by the variance in perceived mixed land use and the availability of roads and walking paths. Eight percent of the variance in exercise time was explained by variance in age and an additional 6% by the variance anxiety/depression. The availability of recreational space added 8%. Six percent of variance in time spent sedentary was explained by family income, while availability of roads and walking paths added another 6%. Conclusions: This study shows the relevance of availability of roads and walking paths and recreational space for more physical activity and less sedentary behavior in people with mental illness. This is particularly relevant in low-income countries where a rapid urbanization is taking place.