This qualitative study focused on the role of teacher agency and critical reflection in the context of an in-service training programme for early childhood education and care (ECEC) teachers in Finland. The programme emphasised culturally sustainable pedagogy and aimed to support teachers in an increasingly multilingual society. This study aimed to investigate how teachers displayed agency as they tried to promote culturally sustaining pedagogy, and how critical reflection interacted with teachers’ professional agency. The data of the study were versatile, as they consisted of influential texts implemented via the creative writing process, two questionnaires, and interviews with the participants. The data were analysed theoretically and empirically through inductive-deductive reasoning. The investigation revealed four levels at which in-service teachers manifest agency in culturally sustaining pedagogy: individual, child-oriented, institutional, and societal. The results revealed that by writing influential texts, ECEC practitioners significantly challenged their assumptions about their agentic capacity and inspired them to reflect on their practices critically. The findings also revealed that in terms of the targeted audience, participants were not confident in their agentic role in influencing systemic change. These findings highlight the importance of discussing multiple aspects of agency in teacher education by using guided critical reflection.