Overactive RAS signaling is prevalent in juvenile myelomonocytic leukemia (JMML) and the myeloproliferative variant of chronic myelomonocytic leukemia (MP-CMML) in humans, and both are refractory to conventional chemotherapy. Conditional activation of a constitutively active oncogenic Nras (Nras^G12D/G12D) in murine hematopoietic cells promotes an acute myeloproliferative neoplasm (MPN) that recapitulates many features of JMML and MP-CMML. We found that Nras^G12D/G12D-expressing HSCs, which serve as JMML/MP-CMML–initiating cells, show strong hyperactivation of ERK1/2, promoting hyperproliferation and depletion of HSCs and expansion of downstream progenitors. Inhibition of the MEK pathway alone prolonged the presence of Nras^G12D/G12D-expressing HSCs but failed to restore their proper function. Consequently, approximately 60% of Nras^G12D/G12D mice treated with MEK inhibitor alone died within 20 weeks, and the remaining animals continued to display JMML/MP-CMML–like phenotypes. In contrast, combined inhibition of MEK and JAK/STAT signaling, which is commonly hyperactivated in human and mouse CMML, potently inhibited human and mouse CMML cell growth in vitro, rescued mutant Nras^G12D/G12D-expressing HSC function in vivo, and promoted long-term survival without evident disease manifestation in Nras^G12D/G12D animals. These results provide a strong rationale for further exploration of combined targeting of MEK/ERK and JAK/STAT in treating patients with JMML and MP-CMML.