In the field of organizational management, research on employees’ life experiences is insufficient. It remains unclear how employees’ non-work experiences affect customer service quality. Building on the spillover theory (an individual’s experience in one domain can be transferred to another domain) and the conservation of resources theory (individuals are motivated to protect their current resources and to acquire new resources), we aim to examine the impact of service employee’s life satisfaction on customer satisfaction while considering service-oriented organizational citizenship behavior (the discretionary extra-role behavior demonstrated by employees) as a mediating variable. To test our hypothesis, we collected multi-wave, multilevel, multisource data of 209 customer service employees from an insurance company. The results showed that service-oriented organizational citizenship behavior was an important mechanism by which employees’ overall life satisfaction positively affected customer satisfaction. In addition, this mediating effect was weaker for employees in higher competition climate groups. The study illustrates the critical contribution of the overall well-being of personnel to organizational business results.