This paper explores the complex problem of project sustainability, focusing on the leadership of three primary school mathematics leaders. Using cultural-historical activity theory (CHAT), the leaders' efforts are reported, highlighting their contribution to project sustainability. The CHAT-informed research design supported the generation of findings, revealing how the mathematics leaders enacted a form of resourceful practice. This paper contributes new knowledge about mathematics leaders, characterising how they acted as agents of project sustainability. Implications for mathematics education project design are also offered.