Myeloproliferative neoplasms (MPNs) cause the over-production of blood cells such as erythrocytes (polycythemia vera) or platelets (essential thrombocytosis). JAK2 V617F is the most prevalent somatic mutation in many MPNs, but previous modeling of this mutation in mice relied on transgenic overexpression and resulted in diverse phenotypes that were in some cases attributed to expression level. CRISPR-Cas9 engineering offers new possibilities to model and potentially cure genetically encoded disorders via precise modification of the endogenous locus in primary cells. Here we develop "scarless" Cas9-based reagents to create and reverse the JAK2 V617F mutation in an immortalized human erythroid progenitor cell line (HUDEP-2), CD34+ adult human hematopoietic stem and progenitor cells (HSPCs), and immunophenotypic long-term hematopoietic stem cells (LT-HSCs). We find no overt in vitro increase in proliferation associated with an endogenous JAK2 V617F allele, but co-culture with wild type cells unmasks a competitive growth advantage provided by the mutation. Acquisition of the V617F allele also promotes terminal differentiation of erythroid progenitors, even in the absence of hematopoietic cytokine signaling. Taken together, these data are consistent with the gradually progressive manifestation of MPNs and reveals that endogenously acquired JAK2 V617F mutations may yield more subtle phenotypes as compared to transgenic overexpression models.