This research addresses an important problem related to Indigenous and low Socioeconomic status (SES) disadvantage in education and the increasing numbers of Indigenous and low SES young people involved in detention in the juvenile justice system. These groups have historically underachieved in mathematics. They continue to experience lower than expected achievement in the subject which limits their engagement in learning and future educational opportunities, and this reality presents new challenges for teachers. The project draws on two conceptual frameworks to advance knowledge about mathematics teaching and incarcerated and low SES young people’s learning. It is based on decolonising methodology in that it collaborates with Indigenous and low SES young people and teachers and focuses on improving learning outcomes (ensuring that the researched benefit). The researchers conducted a cohort study involving one hundred and twenty incarcerated young people and a mathematics teaching team of ten teachers who participated in the first eighteen months of a three-year study. Students were administered mathematics diagnostic tasks before and at the completion of the first year of the study, participated in focus group interviews and completed an efficacy survey. Teachers participated in lesson modelling, semi-structured interviews, focus group interviews and were also administered an efficacy survey. Students indicated their preferred approach to learning mathematics and the types of support they required. Teachers provided a range of instructional strategies to support students and included making reasonable adjustments and differentiation in their instruction. Teachers were identified as drawing on a range of curricula to support the young people. Such curricula included the Australian Core Skills Framework and the Basic Key Skill Builder.