The current study investigates the impact of board gender diversity (BGD) on family firms’ (FFs) engagement in corporate environmental responsibility (CER) practices. The role of BGD in CER policies has been explored in light of critical mass theory by identifying the threshold of women share on board that can influence the environmental commitment of FFs. By employing a fixed-effect (FE) regression analysis on a global sample of 171 FFs, over the 2015-2020 study period, our findings show that when BGD reaches a certain threshold, i.e. critical mass, the CER engagement of FFs increases. These evidences advance prior literature on the link between BGD and CER while providing additional indications for managers, policy makers and FFs seeking the best CER performance.