Leukemia-driving mutations are thought to arise in hematopoietic stem cells (HSC), yet the natural history of their spread is poorly understood. We genetically induced mutations within endogenous murine HSC and traced them in unmanipulated animals. In contrast to mutations associated with clonal hematopoiesis (such as Tet2 deletion), the leukemogenic KrasG12D mutation dramatically accelerated HSC contribution to all hematopoietic lineages. The acceleration was mediated by KrasG12D-expressing multipotent progenitors (MPP) that lacked self-renewal but showed increased proliferation and aberrant transcriptome. The deletion of osteopontin, a secreted negative regulator of stem/progenitor cells, delayed the early expansion of mutant progenitors. KrasG12D-carrying cells showed increased CXCR4-driven motility in the bone marrow, and the blockade of CXCR4 reduced the expansion of MPP in vivo. Finally, therapeutic blockade of KRASG12D spared mutant HSC but reduced the expansion of mutant MPP and their mature progeny. Thus, transforming mutations facilitate their own spread from stem cells by reprogramming MPP, creating a preleukemic state via a two-component stem/progenitor circuit.