Mass participation events are promoted in South Africa as a positive public health initiative. Parkrun has grown to be one of the most popular. The present study identifies the motives of residents in the Western Cape Province to join parkrun and how their involvement influences future physical activity levels. Participants (N = 1787) completed a survey consisting of demographic history, parkrun participation history, motivations for participation, and physical activity-related behaviour changes associated with parkrun participation. The majority of participants were female (n = 952) and over 50 years of age (median = 50; IQR = 38–59). Along with health-related benefits, the provision of a safe and organised event was reported as a key motive to participate. The social connectedness developed by parkrun encouraged continued participation and promoted uptake of more physical activity. Close to half the participants reported increases in physical activity levels after joining parkrun, which demonstrates the benefit obtained from participation in structured mass participation events. With the large diversity in socioeconomic status in South Africa related to physical activity levels, parkrun provides a protected and engaging environment that provides opportunity for increased physical activity and potentially reducing the burden on the healthcare system.