For identification, we exploit cross-sectional variations in opportunities for women in states where directors went to college and temporal variations in discrimination against women when directors were college age, as well as the California law change in 2018 mandating female directors for firms headquartered in California. In terms of channels, we show that female directors bring more expertise on sustainability in boardrooms than male directors, that female directors are more likely to sit on sustainability committees and key monitoring committees than male directors, and that boards with more female directors are more likely to link top executives’ compensation to corporate ESG performance. We conclude that there are important environmental benefits for boards to be gender diverse.