Teachers' professional development is considered a key to school improvement. Unfortunately the quality of primary schooling in Pakistan, especially in rural areas, has not been good due to the traditional practices of teachers. There have been many failed efforts to bring about sustainable improvement in teaching and learning in primary schools. Hence, sustainability of change has emerged as an issue. The present government encouraged school improvement through capacity-building in selected districts of Sindh and Balochistan under the ESRA project initiatives. One-year cluster-based mentoring programs were offered in these selected districts. Coming from one of the districts, my intention was to research the sustainability of impact of one of these one-year cluster-based mentoring program. Focusing on the one-year cluster-based mentoring program, perspectives of the different stakeholders were studied. The findings show that the cluster-based mentoring program had a positive impact on the practices of teachers. They improved their usage of innovative teaching strategies, created a learning environment in class, and developed confidence and learning attitude. However, after the completion of the program, they eventually left these practice due to many long-existing challenges and hindrances in the context. These hindrances include lack of follow-up, resistance by teachers, management, parents, teachers' low economical status, political interference, and so on. Participants gave many suggestions to address these issues in order to sustain the program, such as continuous professional development mechanism, improvement in management practices, provision of resources, and eradicating political influence in education. Finally, the conclusion has been presented and implications for different stakeholder in teacher education in Pakistan are discussed and suggestions are given for further research.