This study examines the relationship between managerial gender diversity and firm performance. It outlines how extremely low and extremely high levels of managerial gender diversity can trigger group processes that can impede the attainment of the performance benefits associated with moderate levels of managerial gender diversity. Findings from a longitudinal panel data from financial service firms in Portugal suggest the effects of managerial gender diversity on firm performance are best captured by a nonlinear function with two breaking points. This study introduces a framework that combines different theoretical perspectives focused on tokenism, sub-group formation, divergent thinking, and other group processes linked to positive and negative gender-diversity consequences. Corresponding overall firm-performance outcomes are contingent upon the level of managerial gender diversity.