Secondary acute myeloid leukaemia complicating the natural disease course of pre-existing Philadelphia chromosome-negative myeloproliferative neoplasms (PN-MPN) is well documented and associated with treatment challenges and significant morbidity. The incidence of a T-cell malignancy developing in patients with pre-existing PN-MPN is uncommon, with one case documented in the literature. We present two cases of angioimmunoblastic T-cell lymphoma (AITL) and one case of T-cell acute lymphoblastic leukaemia (T-ALL) that developed in patients with essential thrombocythemia (ET) and primary myelofibrosis (PMF), respectively. All malignancies were advanced at diagnosis and exhibited disease progression, regardless of the mutational status of the underlying ET/PMF, presence of cytogenetic abnormalities, type of T-cell neoplasm or systemic chemotherapy utilised. The median time to diagnosis of AITL or T-ALL from the onset of MPN was 4.5 years (range: 6 months–10 years). This single institutional case series demonstrates the possibility of an association between T-cell neoplasms and PN-MPNs.