Chronic diseases—including cancer, cardiovascular diseases, and metabolic conditions such as diabetes and obesity—account for over 60% of overall global mortality. Sedentary time increases the risk for chronic disease incidence and mortality, while moderate to vigorous physical activity is known to decrease risk. Most Americans spend at least half of their time sedentary, with a trend toward increasingly sedentary lifestyles, and few Americans achieve recommended levels of physical activity. Time spent outdoors has been associated with reduced sedentary time and increased physical activity among children/youth and the elderly, but few population-based studies have examined this relationship among working age adults who may face greater constraints on active, outdoor time. This study examines the relationship between time spent outdoors, activity levels, and several chronic health conditions among a population-based sample of working age American adults in the National Health and Nutrition Examination Survey (NHANES) for 2009–2012. Findings provide evidence that time spent outdoors, on both work days and non-work days, is associated with less time spent sedentary and more time spent in moderate to vigorous physical activity. Further, findings indicate that time spent outdoors is associated with lower chronic disease risk; while these associations are partially explained by activity levels, controlling for activity levels does not fully attenuate the relationship between time outdoors and chronic disease risk. While cross-sectional, study findings support the notion that increasing time spent outdoors could result in more active lifestyles and lower chronic disease risk. Future work should examine this relationship longitudinally to determine a causal direction. Additional work is also needed to identify mechanisms beyond physical activity, such as psychosocial stress, that could contribute to explaining the relationship between time spent outdoors and chronic disease risk.