Based on legitimacy theory and upper echelons theory, this empirical study addresses whether sustainability expertise in Corporate Sustainability Committees (CSCs) has an impact on firms’ environmental performance. Moreover, a moderator analysis of the implementation of Chief Sustainability Officers (CSOs) on that link is included. Panel regressions are conducted for a sample of STOXX Europe 600 firms for the business years 2016–2018 (918 firm-year observations). While sustainability expertise in CSCs significantly increases firms’ environmental performance, the link is more pronounced by the implementation of CSOs. Thus, CSC sustainability expertise and CSO existence are classified as complementary sustainability board composition variables. The results contribute to the current European regulations on “green” finance and complement prior empirical research on the impact of sustainability board expertise on firms’ environmental performance.