Although teacher agency is increasingly recognised by educationists in the Global South, it is not always well understood or accommodated in policy, as experienced with Universal Primary Education (UPE) reforms in Uganda. This empirical study analyses its role on the UPE reforms, the factors that shape it and its implications for curriculum making. The study adopted a qualitative case study of two UPE schools, utilising an ecological approach to understanding teacher agency. It found that UPE teachers in the selected schools were strongly achieving agency in their daily practice through different forms of 'resistance', 'mitigation' and 'cooperation'. Their agency is being shaped by ecological factors comprising of their temporal dimensions mediated by the affordances and constraints of their cultures, structures and materials, with significant implications. The study concludes that stakeholders need to better explicate teacher agency in policy and scholarship.