Mutations in the DYNAMIN2 (DNM2) gene are frequently detected in human acute T-cell lymphoblastic leukemia (T-ALL), although the mechanisms linking these mutations to disease pathogenesis remain unknown. Using an ENU-based forward genetic screen for mice with erythroid phenotypes, we identified a heterozygous mouse line carrying a mutation in the GTPase domain of Dnm2 (Dnm2V265G) that induced a microcytic anemia. In vitro assays using the V265G mutant demonstrated loss of GTPase activity and impaired endocytosis that was comparable to other DNM2 mutants identified in human T-ALL. To determine the effects of DNM2 mutations in T-ALL, we bred the Dnm2V265G mice with the Lmo2 transgenic mouse model of T-ALL. Heterozygous Dnm2 mutants lacking the Lmo2 transgene displayed normal T-cell development, and did not develop T-ALL. In contrast, compound heterozygotes displayed an accelerated onset of T-ALL compared with mice carrying the Lmo2 oncogene alone. The leukemias from these mice exhibited a more immature immunophenotype and an expansion in leukemic stem cell numbers. Mechanistically, the Dnm2 mutation impaired clathrin-mediated endocytosis of the interleukin (IL)-7 receptor resulting in increased receptor density on the surface of leukemic stem cells. These findings suggest that DNM2 mutations cooperate with T-cell oncogenes by enhancing IL-7 signalling.