Extending research on the performance of Mergers and Acquisitions (M&As), this paper seeks to explain how the post-acquisition integration phase affects acquisition performance. Despite extensive research efforts, there remains a scant understanding of how acquisition implementation, particularly in the post-acquisition integration phase, impacts the performance of M&As. Based on an extensive study of eight acquisitions, in this paper, a grounded model detailing the mechanisms by which the post-acquisition integration phase affects acquisition performance is developed. The model posits that integration-related factors do not bear directly upon acquisition performance. Instead, their effect is mediated by functional organizations in both firms. When focusing into these functional mediating dynamics, we observe that integration-related processual, behavioural and cultural factors affect the identified functional mediators in different ways. Going forward, we echo calls for integrated perspectives to the study of M&A and M&A performance in particular.