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Immune checkpoint inhibitor immunotherapy enhances T cell activity against cancer cells but often leads to the immune system targeting healthy cells, causing ‘immune-related adverse events’ (irAEs). We report the case of a man in his 70s with metastatic melanoma being treated with ipilimumab and nivolumab. He presented with febrile neutropenia after lacerating a thumb on his compost bin. The neutrophil count only recovered once oral prednisolone was commenced, indicating a haematological irAE. A restaging Positron Emission Tomography scan revealed intramuscular ring-enhancing collections throughout his body, one of which was aspirated and cultured the fungus Lomentospora prolificans. He was treated with voriconazole plus terbinafine, both of which were later substituted with olorofim due to a voriconazole-induced rash. After 4 months, the collections had resolved on repeat imaging, and olorofim was ceased due to transaminitis. This case highlights neutropenia as a lesser-known adverse effect of immunotherapy and raises awareness about L. prolificans infection.