We examine the impact of board-management commonality on firm value and board effectiveness. Using the classification properties of a support vector machine (SVM) to measure board-management commonality, we find a positive effect of commonality on firm value. The result is robust to using an instrumental approach, an analysis that exploits the 2019 enactment of Illinois Public Act 101-0589 as an exogenous shock, unexpected deaths of directors and managers, and a placebo test. The positive effect is particularly pronounced in firms with diverse leadership and those facing high industry and market uncertainties. Additionally, commonality enhances board decision-making effectiveness and innovation outputs.