One of the key challenges for firms is to manage sustainability along the supply chain. To extend sustainability to suppliers, organizations have developed different governance mechanisms. The aim of this paper is to analyze the effectiveness of two different mechanisms (i.e., supplier assessment and collaboration with suppliers) to improve one dimension of sustainability: environmental performance. Structural Equation Modeling and cluster analysis were used to analyze the relationships between supplier assessment, collaboration with suppliers, and environmental performance. The results suggest that (1) both mechanisms, supplier assessment and collaboration with suppliers, have a positive and synergistic effect on environmental performance, and (2) assessment acts as an enabler of collaboration. Finally, the paper also contributes to the literature by providing a framework of sustainability governance mechanisms.