Green parties have unique ownership over environmental and climate protection issues in party competition. However, it remains controversial, whether they truly promote climate protection policies, that in turn affect countries' environmental protection achievements. In this paper, I address this question by analyzing the effect of green parties' government participation on policy outputs and countries' environmental protection performance. To this end I adopt a comparative approach, analyzing a large sample of European countries that have or not have been governed by green parties in the past. To study green parties' overall impact, I not only focus on governments' policy outputs but also countries' environmental protection performance. Taking into account two different dependent variables yields more general empirical findings and a more cohesive substantial analysis. Methodologically, I follow a multi-method approach combining classical regression analysis with synthetic control methods. My findings have important implications for the literature on party competition on climate protection, issue ownership, and more generally the role of green parties in legislative democracies.