This study examines the impact of board busyness (i.e. multiple directorships of outside board members) on the performance and financial stability of banks in a dual banking system (Islamic and conventional). We consider banks from 14 countries for the period 2010–2015. The results provide strong evidence that conventional banks with busy boards exhibit high bank performance (i.e. high profitability and low cost to income) and greater financial stability (i.e. low insolvency risk, credit risk, liquidity risk, asset risk, and operational risk). These findings are in line with the reputation hypothesis, which asserts that the expertise and connections of busy outside directors lead to better decision making, more efficient resource utilisation and more effective monitoring. In contrast, Islamic banks’ performance and stability are adversely affected by the presence of busy board members, with Islamic banks show low profitability, high cost to income and high risk-taking. This result might be attributed to the complex governance structure of Islamic banks and the uniqueness of their financial products, which require additional effective monitoring.