Juvenile myelomonocytic leukaemia (JMML) is an aggressive myeloproliferative neoplasm in children characterized by granulocyte macrophage colony-stimulating factor (GM-CSF) hypersensitivity and resistance to chemotherapy. We recently identified c-Cbl (henceforth referred to as Cbl) as a GM-CSF receptor (GMR) responsive protein that targets Src for ubiquitin-mediated destruction upon GM-CSF stimulation and showed that a loss of negative regulation of Src is pivotal in the hyperactivation of GMR signalling in JMML cells. However, the mechanism regulating the chemoresistant nature of JMML has remained largely unknown. Here, we show that the JMML-associated Cbl mutant in complex with the Src family kinase Lyn promotes Cbl's adapter function, leading to increased association to PI3K regulatory subunit p85 and Lyn-dependent AKT pro-survival signalling. Notably, molecular or pharmacologic inhibition of the Lyn-PI3K/AKT pathway, but not the Ras/mitogen-activated protein kinase signalling axis, markedly increased the sensitivity of the otherwise chemoresistant Cbl mutant-JMML cells to chemotherapeutic agents currently used in the treatment of JMML patients. These results support the potential translational benefit of combining modalities that inhibit Lyn-PI3K/AKT signalling with traditional antileukaemia agents in the management of JMML.