T-cell lymphoblastic lymphoma (T-LBL) is a heterogeneous malignancy of lymphoblasts committed to T-cell lineage. The dismal outcomes (15%-30%) after T-LBL relapse warrant establishing risk-based treatment. To our knowledge, this study presents the first comprehensive, systematic, integrated, genome-wide analysis including relapsed cases that identifies molecular markers of prognostic relevance for T-LBL. NOTCH1 was identified as the putative driver for T-LBL. An activated NOTCH/PI3K-AKT signaling axis and alterations in cell cycle regulators constitute the core oncogenic program for T-LBL. Mutated KMT2D was identified as a prognostic marker. The cumulative incidence of relapse was 47% ± 17% in patients with KMT2D mutations, compared with 14% ± 3% in wild-type KMT2D. Structural analysis of the mutated domains of KMT2D revealed a plausible impact on structure and functional consequences. These findings provide new insights into the pathogenesis of T-LBL, including high translational potential. The ongoing LBL 2018 trial (www.clinicaltrials.gov #NCT04043494) allows for prospective validation and subsequent fine tuning of the stratification criteria for T-LBL risk groups to improve survival of pediatric patients.