This work investigated the impact of corporate governance on employee productivity with special focus on gender diversity in selected Nigerian local banks whose operating licences predated 2006. The study adopted quantitative approach; applied descriptive and inferential statistical analyses; and extracted data from the sampled banks’ audited annual accounts from 2012 to 2016. Four hypotheses were developed in line with reviewed literature, and tested using multiple regression analysis and univariate general linear model. The study anchored on critical mass and upper echelon theories. The results reveal that: (i) board gender diversity has significant positive effect on Nigerian banks’ employee productivity; (ii) top-management gender diversity does not have significant effect on Nigerian banks’ employee productivity; (iii) board chairman gender diversity has significant positive effect on Nigerian banks’ employee productivity; and (iv) board secretary gender diversity does not have significant effect on Nigerian banks’ employee productivity. The practical implications of this research are that Nigerian banks should promote board gender diversity and board chairmen gender diversity to increase their employee productivity. Theoretically, this research establishes the importance of critical mass theory on Nigerian banking sector: board gender diversity has significant effect on employee productivity.