Langerhans cell histiocytosis (LCH) is a rare type of histiocytosis with various clinical manifestations. The differential diagnosis for an infiltrative process in the mesentery includes histiocytosis, such as Erdheim–Chester disease (ECD) and Rosai–Dorfman disease (RDD), but mesenteric involvement has never been reported. Here we present the first documented case of LCH. A 24-year-old man presented with diabetes insipidus and was diagnosed with LCH via mesenteric biopsy, which demonstrated mesenteric involvement and the BRAFN486_P490del mutation. He was treated with trametinib 2 mg once daily, and FDG-PET/CT at 6 months after treatment initiation showed complete remission. Our report suggests that trametinib is a promising treatment option for non-BRAFV600E mutant LCH with mesenteric involvement.