Post-transplant lymphoproliferative disorders (PTLD) and lymphomas in immunocompromised individuals represent significant clinical challenges, with a limited understanding of their pathogenesis. We investigated a PTLD cohort (n = 50) consisting of ‘early lesions’ (infectious mononucleosis-like PTLD, plasmacytic and follicular hyperplasias), polymorphic PTLD and post-transplant diffuse large B-cell lymphomas (PT-DLBCL). The study also included 15 DLBCL with autoimmune/immunocompromised backgrounds (IS-DLBCL) and 14 DLBCL, not otherwise specified (DLBCL, NOS), as control. To investigate microarchitectural and genetic changes, immunohistochemistry, multiplex immunofluorescence (mIF), fluorescence in situ hybridisation and high-throughput sequencing were performed. Scarcity of viral infections other than Epstein–Barr virus (EBV) was observed. mIF revealed lower Treg infiltration in PT-DLBCL and high CD8⁺/PD1⁺ T cells in IS-DLBCL. MYC rearrangements were most common in PT-DLBCL, followed by IS-DLBCL and DLBCL, NOS, all EBV-negative. TP53 mutations were frequent in EBV-negative PT-DLBCL and DLBCL, NOS but absent in ‘early lesions’. NOTCH1 mutations were predominant in PT-DLBCL (N1 DLBCL-subgroup). Gene expression profiling showed a significant overlap between ‘early lesions’ and polymorphic PTLD. The presence of clonal haematopoiesis of indeterminate potential (CHIP)-like mutations and the absence of immune-escape gene mutations in ‘early lesions’ suggest these disorders may represent clonal expansions driven by exogenic immunosuppression and/or EBV infection ‘substituting’ for mutations of the latter group of genes.