A growing body of research suggests that the composition of a firm’s board of directors can influence its environmental, social and governance (ESG) performance. In the banking industry, ESG performance has not yet been explored to discover how a critical mass of women on the board of directors affects performance. This paper seeks to fill this gap in the literature by testing the impact of a critical mass of female directors on ESG performance. Other board characteristics are accounted for: independence, size, frequency of meetings and Corporate Social Responsibility (CSR) committee. We use fixed effects panel regression models on a sample of 108 listed banks in Europe and the United States for the period 2011–2016. Our main empirical evidence shows that the relationship between women on the board of directors and a bank’s ESG performance is an inverted U-shape. Therefore, the critical mass theory for banks is not supported, confirming that only gender-balanced boards positively impact a bank’s performance for sustainability. There is a positive link between ESG performance and board size or the presence of a CSR committee, while it is negative with the share of independent directors. With this work, we stress the key role of corporate governance principles in banks’ ESG performance, with relevant implications for both banks and supervisory authorities.