Using Baltagi and Li's (2002) semi-parametric panel fixed effects model, this article investigates the effect of board gender diversity (BGD) on a firm's environmental performance (EP) in a sample of firms from the 2020 Fortune 1000 Index over the 2004–2020 period. We address the endogeneity issue via a control function. Contrary to the existing literature, we find an inverted U-shaped BGD-EP relationship with a turning point. Similarly, we show that this relationship is endogenous and dynamic by nature. Overall, we find a curvilinear relationship (inverted U-shape) between BGD and EP, thereby supporting the “too-much-of-a-good-thing” (TMGT) effect (Pierce and Aguinis, 2013).