The client relationships and tacit knowledge of professionals are professional service firms' (PSFs') major value creating resources producing challenges in generating post‐merger and acquisition value and risks of reducing the productivity or losing key professionals and their clients. The recent emergence of publicly owned PSFs and rapid growth through consolidating smaller privately owned firms potentially increases this risk by integrating small firms and their professionals into large public companies that may be governed very differently. This study explores post‐acquisition integration processes: professional behaviours and associated performance implications in two highly acquisitive publicly owned accounting companies. In one company, the integration process was rapid and heavily directed by senior management, while in the second company, integration was more gradual, initially undirected but then facilitated by senior management. The findings suggest that integration processes can impact the behaviour of professionals and acquiring firm performance. This research contributes to the understanding of post‐acquisition integration processes in PSFs and decision‐making and professional behaviour in recently emerged publicly owned PSFs. The study contributes to knowledge‐based theory by identifying factors that can affect the decision authority of executives and how organisational behaviour can constrain the implementation of executive integration decisions. Copyright © 2017 John Wiley & Sons, Ltd.