The increasingly widespread use of immunohistochemistry and next-generation sequencing (NGS) in the detection of microsatellite instability (MSI) and DNA mismatch repair (MMR) status has led to the observation of various unusual tumour types that exhibit MMR protein deficiency in Lynch syndrome (LS). Here, we report a case of two synchronous colorectal cancer (CRC) tumours simultaneously occurring in a 42-year-old woman with a deleterious germline mutation in MSH6, abundant expression of PD-L1 and high tumour mutation burden (TMB). The two CRC tumours (tumours A and B) harboured highly heterogeneous features. One showed loss of MSH6 protein and a microsatellite stable (MSS)/MSI-low (MSI-L) status, while the other presented no loss of MMR protein and MSI-H status. Furthermore, the 9 common mutated genes between the two CRC tumours had no shared mutation sites. Only 4 KEGG pathways were identified as enriched for five of the common mutated genes, while 8 cancer-related pathways were identified as enriched for 9 and 13 unique mutated genes in tumours A and B, respectively. Therefore, we chose immune checkpoint inhibitors (ICIs) as the potential therapy. This case exemplifies the complexity of tumorigenesis and potential ICI treatment in LS patients.