Although preschool principals are important for integrating sustainability, studies on this topic are scarce. Using an embedded mixed methods approach, this study explored and compared 50 principals' views and actions relating to sustainability in the operations of 25 eco- and 25 non-eco-certified randomly sampled preschools from 25 municipalities (of a total 290) in Sweden. Using a semi-structured questionnaire, data was collected from the principals of the preschools. Whole School Approach Flower Model was used as an analytical framework to analyze qualitative data, while quantitative data was subject to Orthogonal Projections to Latent Structures Discriminatory Analysis. According to the participating principals, eco-certified preschools focused on increasing children's knowledge about and interests in sustainability, whereas non-eco-certified preschools focused on strategies to develop teachers' sustainability competence. This is consistent with findings that principals at eco-certified preschools have a more nuanced and multifaceted view of sustainability in their leadership practice, indicating that eco-certification, to some extent, plays an important role in making a difference. The study also highlights the potential of adopting a whole school approach to sustainability within preschool education. Cross-validation supported the generalizability of the conclusions, at least within a Swedish context.