The acquired gain-of-function V617F mutation in the Janus Kinase 2 (JAK2^V617F) is the main mutation involved in BCR/ABL-negative myeloproliferative neoplasms (MPNs), but its effect on hematopoietic stem cells as a driver of disease emergence has been questioned. Therefore, we reinvestigated the role of endogenous expression of JAK2^V617F on early steps of hematopoiesis as well as the effect of interferon-α (IFNα), which may target the JAK2^V617F clone in humans by using knock-in mice with conditional expression of JAK2^V617F in hematopoietic cells. These mice develop a MPN mimicking polycythemia vera with large amplification of myeloid mature and precursor cells, displaying erythroid endogenous growth and progressing to myelofibrosis. Interestingly, early hematopoietic compartments [Lin-, LSK, and SLAM (LSK/CD48−/CD150+)] increased with the age. Competitive repopulation assays demonstrated disease appearance and progressive overgrowth of myeloid, Lin-, LSK, and SLAM cells, but not lymphocytes, from a low number of engrafted JAK2^V617F SLAM cells. Finally, IFNα treatment prevented disease development by specifically inhibiting JAK2^V617F cells at an early stage of differentiation and eradicating disease-initiating cells. This study shows that JAK2^V617F in mice amplifies not only late but also early hematopoietic cells, giving them a proliferative advantage through high cell cycling and low apoptosis that may sustain MPN emergence but is lost upon IFNα treatment.