The balance of hematopoietic stem cell (HSC) self-renewal versus differentiation is essential to ensure long-term repopulation capacity while allowing response to events that require increased hematopoietic output. Proliferation and differentiation of HSCs and their progeny is controlled by cytokine signaling and activation of the JAK/STAT pathway. One mechanism utilized to modulate activity of the JAK/STAT pathway is inhibition of signaling components. E3 ubiquitin ligases are enzymes that add ubiquitin modifications to specific protein substrates, targeting them for recruitment to the proteasome or altering their localization and activity. Cul5 is a scaffold protein that holds together an E3 called Cullin Ring Ligase 5. CRL5 works with Suppressors of Cytokine Signaling proteins as well as other substrate receptors to degrade many different protein substrates in a cell type and context specific manner. We sought to uncover the role of Cul5 in the function of hematopoietic stem cells. Here we report that mice lacking Cul5 in hematopoietic cells (Cul5Vav-Cre) have increased numbers of hematopoietic stem and progenitor (HSPCs), splenomegaly, and extramedullary hematopoiesis. Differentiation in Cul5Vav-Cre mice is myeloid- and megakaryocyte-biased, resulting in leukocytosis, anemia and thrombocytosis. Cul5-deficient HSPCs have impaired bone marrow homing, possibly due to CXCR4 downregulation, resulting in increased HSPCs in circulation in Cul5Vav-Cre mice. In HSPCs, Cul5 associated with STAT5 as well as nine different substrate receptors including several SOCS proteins, and lesser studied WSB1 and LRRC41. LRRC41 and pSTAT5 accumulated in Cul5Vav-Cre HSCs during the course of IL-3 stimulation. Inhibition of the proteasome by bortezomib treatment on WT HSPCs stimulated with IL-3 increased pSTAT5 levels over untreated cells. Together, these data support that Cul5 and LRRC41 form a CRL5 complex in HSPCs and that Cul5 inhibits activity of pSTAT5 in a proteasomal dependent manner. Whole cell proteome (WCP) analysis of HSPCs from Cul5Vav-Cre bone marrow showed upregulation of many STAT5 target genes and associated pathways. Finally, treatment with ruxolitinib, a JAK1/2 inhibitor, normalized hematopoiesis in Cul5Vav-Cre mice. These studies demonstrate a novel function of Cul5 in HSC function, stem cell fate decisions, and JAK/STAT regulation that may facilitate new therapies for hematological diseases.