This study aimed to explore the amenable factors contributing to the improvement in subjective well-being (SWB) immediately after a short-term visit to an urban park in an uncontrolled condition. Ninety-four park visitors from three urban parks completed a short questionnaire evaluating SWB (with two components: affect and life satisfaction) immediately before and after their park visit. In addition, their level of physical activity was tracked by wearing an accelerometer during the park visit. Results indicated a significant improvement in SWB, affect, and life satisfaction scores of park visitor participants from before and after their visit. Duration of park visit was bivariately associated with SWB scores, and independently associated with the improvement in life satisfaction scores, controlling for parks and age, after the visit; a 20.5-min park visit predicted the highest overall accuracy (64%) improvement in life satisfaction. It is recommended that design of the park space should attract visitors to stay for at least 20 min in the park.