In a survey of key account managers, we examine how internal and external collaboration individually and jointly affect performance. We shed light on the role of proactiveness in generating these effects. Using a social exchange theory lens, we consider how managing a portfolio of relationships through collaboration, both inside and outside of the organization, plays an important role in key account manager performance. Using data collected from key account managers across a range of industries, this study finds that proactive key account managers have higher levels of collaboration and performance. We find however that only the joint effect of internal and external collaboration positively affects performance. Interestingly, there were no direct effects. This study extends our understanding of the nature of collaboration in business-to-business relationships. Drawing on these findings, we discuss several theoretical and managerial implications.