This paper aims at investigating organizational mechanisms through which firms involved in open innovation initiatives can acquire external knowledge, integrate it with the existing one residing in the diverse functional areas, and transform it into innovation outcomes. Following the knowledge transformation perspective, we use the notions of early‐stage and late‐stage functional involvement, and explain their mediating effects on a firm's innovation performance. Based on a sample of 131 international firms involved in open innovation projects, we find that high involvement of functions related to the early stage of the innovation process – notably new concept generation, research and development, and design and testing – fully mediates the effect of external knowledge transfer on innovation performance. Similarly, high involvement of functions related to the late stage of the innovation process – notably manufacturing, marketing, distribution, and logistics – has significant indirect effect on innovation performance but lower than that of early‐stage functional involvement. Furthermore, the empirical research reveals that early‐stage functional involvement mediates the positive effect of external knowledge transfer on late‐stage functional involvement. Theoretical and practical implications of our findings are discussed.