Current in vitro models of developmental blood formation lack spatiotemporal coherence and weakly replicate the hematopoietic microenvironment. Developmentally-appropriate models can enhance understanding of infant acute myeloid leukemia (infAML), which putatively originates in utero and has 50% age-unique genetic events, suggesting unique biology. The commonest genetic abnormality unique to infants involves homeobox gene MNX1, whose leukemogenic mechanisms remain unknown. Recently, 3D self-organising embryonic stem cell (SC)-based gastruloids have shown promise in recapitulating embryonic events with time/space precision. Herein, we report a hemogenic gastruloid (haemGx) system that captures multi-wave blood formation, progenitor specification from hemogenic endothelium (HE), and approximates generation of hematopoietic SC precursors. Enforced MNX1 expression in haemGx promotes HE formation, perturbs endothelial-to-hemogenic transition, and critically achieves transformation, generating myeloid colonies which display MNX1 AML signatures. By combining functional assays with single-cell transcriptomics, we establish the haemGx as a new model of normal and leukemic embryonic hematopoiesis amenable to mechanistic exploration.