Purpose This study aims to examine the impact of agency cost, Islamic board characteristics and corporate governance on the performance of Islamic institutions. Design/methodology/approach Based on the selected criteria, 92 Islamic banks (IBs) from 20 countries were selected for further research. The authors used generalized method moments (GMM) estimation method. The agency cost and Shariah board characteristics are the explanatory variables. The author uses the age of the bank and the size of the bank for variable control. Findings Empirical results indicate that first, agency costs represented by cast/total assets negatively affect IBs’ return on equity and net income. As agency costs rise, IBs’ financial performance declines. Second, Shariah supervisory board (SSB) size and board independence affect IB performance. The study found that SSB size positively affects IB performance. Research limitations/implications This research contributes to the literature on IBs in different countries, which policymakers and practitioners can use to improve agency cost functions and Shariah board characteristics. Second, this analysis shows that IBs require specific attention for agency charges, given their operations and business structures. This study contributes to agency theory, which requires Islamic banking information and practices. Finally, the author has aided regulators and IBs by identifying the sources of agency cost practices that can be resolved. The other bank governance contribution is twofold. First, the author studied dual board governance in IBs (SSB and ordinary boards of directors). Second, the author examines how SSB and traditional board governance affect IB performance. This research focuses on banks listed on stock exchanges in the 20 countries analysed. Practical implications The research has policy and practical implications for central banks and IBs. By outlining appropriate regulatory guidelines and reporting systems, regulatory authorities can ensure Sharia compliance and protect the independence of IB Shariah department officers. Regulators and relevant stakeholders must ensure Sharia compliance, audits, inspections, reporting and accurate disclosure for IBs. Originality/value This paper offers original contributions to professionals in the field of IBs and stakeholders investigating the relationship between agency costs, governance of IBs, characteristics of Islamic supervisory boards and the performance of IBs.