This research investigates how pre-service teachers aim to bring awareness of environmental sustainability while learning mathematical modelling to their 5th-grade students. Theoretical perspectives from socio-critical modelling research can involve engaging students in societal issues, including focusing on action. Thematic coding of a mandatory task by a group of pre-service teachers, where they design, implement, and reflect on a modelling activity during practicum, is used to analyse how they combine environmental and mathematical aims. The findings show that the pre-service teachers emphasise the authenticity of tasks and data, engage students in actions in littering, reflect on students’ critique of assumptions, and focus on the meaning-making of environmental numbers using students’ everyday language. Implications to teacher education suggest that teachers can combine focusing on environmental sustainability through mathematics education. Further, teacher education can support pre-service teachers and students in actions through socio-critical modelling activities.