Mastocytosis is a heterogeneous group of diseases with a young median age at diagnosis. Usually indolent and self-limited in childhood, the disease can exhibit aggressive progression in mid-adulthood. Our objectives were to describe the characteristics of the disease when diagnosed among elderly patients, for which rare data are available.The French Reference Center conducted a retrospective multicenter study on 53 patients with mastocytosis >69 years of age, to describe their clinical, biological, and genetic features.The median age of our cohort of patients was 75 years. Mastocytosis variants included were cutaneous (n = 1), indolent systemic (n = 5), aggressive systemic (n = 11), associated with a hematological non-mast cell disease (n = 34), and mast cell leukemia (n = 2). Clinical manifestations were predominantly mast cell activation symptoms (75.5%), poor performance status (50.9%), hepatosplenomegaly (50.9%), skin involvement (49.1%), osteoporosis (47.2%), and portal hypertension and ascites (26.4%). The main biological features were anemia (79.2%), thrombocytopenia (50.9%), leucopenia (20.8%), and liver enzyme abnormalities (32.1%). Of the 40 patients tested, 34 (85%), 2 (5%), and 4 (10%) exhibited the KIT D816V mutant, other KIT mutations and the wild-type form of the KIT gene, respectively. Additional sequencing detected significant genetic defects in 17 of 26 (65.3%) of the patients with associated hematological non-mast cell disease, including TET2, SRSF2, IDH2, and ASLX1 mutations. Death occurred in 19 (35.8%) patients, within a median delay of 9 months, despite the different treatment options available.Mastocytosis among elderly patients has a challenging early detection, rare skin involvement, and/or limited skin disease; it is heterogeneous and has often an aggressive presentation with nonfortuitous associated myeloid lineage malignant clones, and thus a poor overall prognosis.