We focused on mobile engineers, a distinctive employee group that may have unique reactions to mergers and acquisitions (M&As). Mobile engineers, employees that move from one firm to another, were previously recognized as an undesirable loss by most knowledge-intensive organizations. However, in this study, we show that they may return to their former organizations as effective knowledge creators when their previous and new organizations unite through M&As. We specifically investigated how their mobility direction, relational assets, and intellectual assets affect the amount of knowledge that is jointly created through inter-personal collaborations following the M&A. Using the data of 410 mobile engineers in high-technology M&As during 2000–2004 in the United States, we found that the mobility direction from acquiring firms to targets prior to M&A has a positive impact on joint knowledge creation. We also found that such mobility direction positively moderates the relationship between human assets of mobile engineers and their joint knowledge creation.