Therapy-related acute myeloid leukemia and myelodysplastic neoplasms (t-AML/MDS) are devastating complications of chemo- or radiation therapy in patients treated for an unrelated primary malignancy. Cancer patients with TP53-mutant hematopoietic stem and progenitor cells (HSPCs) – a condition termed clonal hematopoiesis (CH) – are at a particularly high risk for t-AML/MDS. However, the pathogenesis of TP53-mutant t-AML/MDS, especially the role of the TP53 allelic state (i.e., mono- vs. biallelic), and its prognostic impact in AML/MDS have remained only poorly understood. We developed novel in vitro and in vivo mouse models to investigate how mono- or biallelic Trp53 mutations influence clonal expansion and leukemic progression from CH to t-AML/MDS. While HSPCs with monoallelic Trp53 mutations gain clonal fitness but retain their genomic integrity under chemo- or radiation therapy, biallelic Trp53 mutations result in genomic instability and are essential for leukemic transformation. Moreover, we provide proof of concept that non-mutational p53 inactivation, such as MDM2 overexpression, can replicate the effects of biallelic TP53 mutations, providing a possible explanation for cases of TP53-mutant AML/MDS that retain one wild-type TP53 allele. Our findings elucidate the pathogenesis of TP53-mutant t-AML/MDS and support the classification of biallelic TP53-mutant AML/MDS as distinct clinical entities.