Significance Isocitrate dehydrogenase 1 ( IDH1 ) mutations are drivers of hematological malignancy. Although these mutations are most often associated with myeloid disease, they are also found in lymphoid malignancies, including T-cell acute lymphoblastic leukemia (T-ALL). Treatment strategies targeting these mutations are currently being devised, including small-molecule inhibitors of the mutant IDH1 enzyme. A better understanding of the role of these mutations in tumorigenesis and their effects on tumor cells will allow these treatment strategies to be effectively translated to the clinic. Here we show that Idh1 mutations can contribute to the development of T-cell malignancies, including T-ALL, using a conditional knock-in mouse model. These mouse models provide a platform for further evaluation of treatment strategies for T-cell malignancy.