This mixed-methods study aims to inform the development of a 'Park Prescription' intervention, including face-to-face counseling on physical activity and park use and providing weekly structured exercise sessions in the park to promote physical activity. Participants aged 40-65 years were recruited from regional health screening events in Singapore where they completed a questionnaire (N = 97) and consented to focus group (FG) participation (N = 16). The questionnaire assessed current park use, and the type, duration, and intensity of park-based activities that would be of interest. FGs explored the barriers and facilitators of physical activity (in parks). Short interviews (N = 16) with 'doers', i.e., people already engaging in park-based physical activity, identified motivational factors and ways to overcome common barriers. Participants acknowledged the health benefits of parks and valued them because of their pleasant landscapes, greenery and facilities. However, few participants engaged in physical activity at the parks, because they were too busy or too tired. Participants mostly indicated doing informal activities, such as walking, cycling or playing traditional Asian games when using the parks for exercise. A variety of low-to-moderate intensity park-based activities such as walking, cycling or aerobics were of interest to participants who expressed the willingness to engage in structured exercise sessions on weekday evenings or weekend mornings. Strategies to increase physical activity in parks included: encourage planning, create social support, identify alternatives for bad weather, improve proximity/accessibility to parks and park safety. The effectiveness of the Park Prescription intervention in promoting physical activity, park use, as well as physical and mental well-being will be tested in a one-year Randomized Controlled Trial.