Chronic myeloid leukemia (CML) and Philadelphia (Ph)-negative myeloproliferative neoplasms (MPN) are generally distinct clonal disorders, with the co-occurrence of BCR::ABL1 rearrangement with concomitant Ph-negative MPN rarely reported. Here we describe the largest known international cohort of Ph-negative MPN and coexisting CML providing important insights into this rare clinical scenario. We performed an international, multicenter, retrospective analysis of patients with concomitant BCR::ABL1 rearrangement and Ph-negative MPN, identifying 61 cases from 30 centers in 7 countries, over a 29-year period (1996-2025). Thirty-one patients (50.8%) had Ph-negative MPN preceding CML, 18 patients (29.5%) had CML preceding Ph-negative MPN, and 12 patients (19.7%) had Ph-negative MPN and CML diagnosed simultaneously. We observed increased TKI resistance and myelofibrotic transformation, especially in patients initially diagnosed with Ph-negative MPN. In this group, 35.4% (n = 11) progressed to MF, 2 patients to blast-phase MPN, and 69.2% (n = 18) failed to achieve a complete cytogenetic response. The rare e1a2 BCR::ABL1 transcript was notably prevalent which is associated with TKI resistance and a more aggressive disease course in CML. We described superior survival in those with Ph-negative MPN preceding CML, with median OS not reached, compared with 277 months for CML preceding Ph-negative MPN and 100 months for those diagnosed simultaneously (p = 0.05).