Existing research on the relationship between human capital and firm performance—a core research topic in the field of strategic human resource management—has mostly focused on human resource management systems, and consequently, research on the direct relationship between the human resources a firm possesses and its performance has been insufficient. This study explored the relationship between a firm's total human resource pool and performance, targeting small and medium-sized manufacturing firms in Korea. In this study, a firm's total human resource pool was conceptualized as the aggregate of human capital and workforce composition, and the relationship between human capital and performance, as well as the effects of the interaction between workforce composition and strategy on performance, were empirically examined. The main research findings are as follows. First, the total human capital of small and medium-sized enterprises has a significant effect on firm performance. Second, the effect of human capital on performance varies depending on strategy. Specifically, when a differentiation strategy is employed, the level of human capital has a positive effect on performance, whereas in firms employing a cost-leadership strategy, the level of human capital has a negative effect on performance. Third, the interaction between workforce composition and strategy has a significant effect on performance. In particular, these results were clearly evident in the cases of cost-leadership strategy and marketing differentiation strategy. Unlike existing research in the strategic human resource management field, which has focused on identifying the relationship between human resource management practices and performance from a systems-centered perspective, this study sought to explore the impact of a firm's total human resources on performance. In particular, the workforce composition variable—which had never been used in prior research—was introduced and utilized in the study, yielding meaningful empirical results. Going forward, along with the development of analytical frameworks for examining firms' human resource pools, diverse perspectives and active discussions are needed regarding the significance of workforce composition—first employed in this study—and approaches to operationalizing this variable.