Amateur athletes invest a lot of effort, time, and money on participating in events such as marathons and bike races. The aim of the study is to explore whether this increases their subjective well-being (SWB). The study addresses both what contributes to SWB and the effects of SWB for participants in large-scale sport events. More than 7,000 sport event participants responded to surveys including measures of sociodemographics, participation in sport events, perceptions of quality and satisfaction of the event, behavioral intentions, and SWB. Building on theories of SWB and serious leisure, hypotheses linked to the causes and effects of SWB are explored. Results show that sociodemographics, past relational activities, and past event participation influence SWB. Furthermore, with higher SWB the sport event participants are more satisfied, perceive a higher quality, and are more likely to participate in future events. These are proposed to be effects of SWB. For sports clubs and event organizers the results help inform marketing efforts before, during, and after events. In particular, if participants with serious leisure careers are identified.