Background: The potent regenerative capability of the vasculature is attributed to populations of resident progenitors, termed endothelial colony forming cells (ECFCs). ECFCs maintain endothelial function and is capable of forming entirely new vessels in vivo, implicating their role in tissue vascular regeneration. However, ECFCs can often acquire mesenchymal phenotype, via a process termed endothelial-to-mesenchymal transition (EndMT). EndMT is thought to contribute to the development of a number of fibrotic diseases, including atherosclerosis. Our study aims to address the pathological catalyst that drives EndMT and delineate its role in the progression of atherosclerosis. Methods and Results: In the presence of excessive oxidised low lipoprotein (ox-LDL), cultured ECFCs demonstrated relative mRNA and protein increase in mesenchymal markers such as Fsp-1, and transcription factors such as Sox9 and Runx2. Long-term treatment of ECFCs with ox-LDL directly contributed to accelerated de-differentiation towards a mesenchymal phenotype and thus a subsequent depletion of the ECFC progenitor pool, as observed under flow cytometry. The self-renewal capacity of ECFCs, assessed through single-cell colony formation assay, was also significantly reduced following 5-days of ox-LDL treatment. This also resulted in the loss of the ability for ECFCs to be passaged in vitro. In addition, RNA sequencing demonstrated significant differences between ox-LDL treated and untreated ECFC. Pathway analysis showed upregulation of mesenchymal markers as well as decreases in key endothelial NOTCH signalling pathway that is essential in maintain ECFC function. Conclusion: The finding of this study supports the bold idea that atherosclerosis is a consequence of vascular resident progenitor depletion and fate choice, representing a paradigm shift from the current understanding and open new avenues for preventive therapies.