Ataxia-telangiectasia (A-T) is an autosomal recessive chromosome breakage disorder caused by mutations in the ATM gene. Typically, it presents in early childhood with progressive cerebellar dysfunction along with immunodeficiency and oculocutaneous telangiectasia. An increased risk of malignancy is also associated with the syndrome and, rarely, may be the presenting feature in small children. We describe a 17-year-old boy with slurred speech, mild motor delays and learning disability diagnosed with atypical A-T in the setting of T-cell acute lymphoblastic leukemia. Suspicion for A-T was raised after review of a peripheral blood karyotype demonstrating rearrangements involving chromosomes 7 and/or 14. The diagnosis was confirmed after molecular testing identified a novel homozygous missense variant in ATM (c.5585T>A; p.Leu1862His) that resulted in protein instability and abolished serine/threonine protein kinase activity. To our knowledge, this is the first report of concurrent A-T and lymphoid malignancy diagnoses in an older child or adult with only mild neurological disease. Our experience suggests that screening for the disorder should be considered in any individual with lymphoid malignancy and neurological findings, especially as radiation and certain chemotherapy protocols are contraindicated in A-T.