The use of external knowledge for innovation (i.e., inbound or outside-in open innovation) has received substantial attention in the innovation literature. However, the “human side” of open innovation is still poorly understood. We consider the role of employee characteristics with respect to predicting firm-level openness. Drawing on the human capital, learning and creativity literatures, we theorize that knowledge diversity of the firm’s employees is positively associated with employees’ ability to identify and absorb external knowledge, which aggregates to increased firm-level openness—that is, firms’ use of external knowledge in their pursuit of innovation. Based on a combination of three data sources, namely, two survey data sources and register data, we find support for our hypothesis that employees’ educational diversity is positively associated with firm-level openness. However, we find no direct association between employees’ work history diversity and firm-level openness but rather—as also hypothesized—a conditional relationship based on educational background, which implies that diverse work history only has a positive impact at higher levels of educational diversity. To reduce endogeneity concerns, we undertake a series of robustness checks.