This study empirically analyzed the impact of non-standard employment on comprehensive firm performance among listed manufacturing companies in Korea from a social exchange theory perspective. The study first introduced theoretical perspectives capable of explaining the relationship between non-standard employment and firm performance—including the learning theory perspective, the fit perspective, and the social exchange theory perspective—and then established research hypotheses from the social exchange theory perspective for empirical analysis. Data obtained through telephone surveys were matched with performance data of listed companies from Korea Investors Service, Inc., forming approximately 182 observations that were subjected to statistical analysis. The results are summarized as follows. First, business strategy was found to partially influence the proportion of non-standard workers and the level of institutionalization of non-standard workforce management. Second, non-standard employment had a dual effect: it reduced labor costs and enhanced numerical flexibility, but increased overall turnover rates and decreased labor productivity and operating profit margins. In particular, the negative relationship between the proportion of non-standard workers and operating profit margin indicates that the belief among managers that expanding non-standard employment for cost reduction purposes can increase ultimate operating profits is unfounded. Third, the level of institutionalization of non-standard workforce management was found to counteract the effects of non-standard employment. If the proportion of non-standard workers in a firm represents the degree of postponement and abandonment of human capital investment, then the level of institutionalization of non-standard workforce management represents the degree of limited investment in human capital directed toward non-standard workers. That is, these two variables can explain the relationship between the firm and its workforce as one of mutual reciprocity based on social exchange theory. Fourth, in terms of the structure of performance outcomes, voluntary turnover rate, labor cost ratio, and labor productivity were found to serve as mediating variables as intermediate outcomes.