In this article, we report on findings from the Primary Maths and Social Justice research project, a recent collaboration between the three authors and six teacher researchers from two culturally diverse primary schools in London, UK. The project aimed to develop strategies for addressing issues of social justice in the mathematics classroom, and operationalising the recently developed theoretical concept of socio-mathematical agency, defined as learners’ capacity to apply mathematics in arguing collectively for social change. The teacher researchers’ experiences, as captured through audio recordings of research team meetings and interviews, form the basis of our data analysis. Our findings highlight the processes that enabled teachers to reflect critically on existing practice, prompting them to adopt more collaborative, discursive and inclusive problem-solving approaches that resulted in significantly higher levels of student engagement in mathematical activities. The teacher researchers found that cross-curricular projects provided more opportunities to apply mathematical knowledge in tackling social justice issues than the prescriptive and content-focused mastery-style mathematics schemes of work in place. They reported how students embraced opportunities to use mathematics in more meaningful contexts and in presenting arguments for change. We argue that a focus on developing socio-mathematical agency has the potential to enhance the role of teachers and learners as agents of change, an important element that is often underdeveloped in culturally responsive teaching.