AbstractThis study explores the institutional field of sustainability assurance in South Africa to explain how this emerging practice, largely embedded in the mature field of auditing, operates. Employing the institutional pillars approach to analyze data obtained from in‐depth interviews, archival sources, and documentary reviews, the study shows mechanisms by which governing norms of the sustainability assurance field in South Africa are maintained despite a conspicuous gap between the desired purpose of sustainability assurance and its actual practice. Building on prior research on capture of sustainability assurance field, we identify an institutional outcome of total—that is, managerial and professional—capture conditioned by (1) malleability of the objective and scope of sustainability assurance engagements and (2) inadequate regulative pillars that failed to manage the multiple and competing interests in the sustainability assurance field. We show that total capture enabled maintaining field norms of sustainability assurance that cater for the interests of assurors and clients with limited regard to enhancing corporate environmental sustainability. The study contributes to the literature by highlighting the institutional outcome of embedding the emerging practice of sustainability assurance in an established field of auditing in a setting with weak regulative pillars.