Background: Mastocytosis is characterized by the accumulation of abnormal mast cells in various organs. Data on the prevalence of mastocytosis are heterogeneous, with the condition’s prevalence estimated to be between 9.6 and 23.9 per 100,000 inhabitants. Patients may present signs and symptoms that can severely impact quality of life (QoL), but reported data are scarce. Thus, we performed a nationwide study to estimate the prevalence and to assess the management and burden of adults with mastocytosis in France according to physician assessments. Methods: We developed an online survey comprising 25 questions investigating various aspects of mastocytosis and asked 6,239 physicians to respond. Data concerning physician characteristics and the number of patients followed were used to estimate overall prevalence. To assess patients’ QoL, we focused on the presence of signs and symptoms and the patients’ burden specifically in those with either indolent systemic mastocytosis (ISM) or mastocytosis in the skin (MIS). Results: Between July 11, 2023, and September 1, 2023, 1,169 physicians (18.7%) completed the survey. These physicians managed 4,121 mastocytosis patients, corresponding to an estimated prevalence of mastocytosis of 8.5 per 100,000 in France. In the ISM/MIS population (representing 76% of mastocytosis patients), 53% presented moderate to severe symptoms (mainly skin, digestive and general symptoms). Overall, physicians indicated that there was substantial burden associated with these symptoms in almost all fields of QoL analyzed. Conclusions: Our results provide further evidence of the burden associated with mastocytosis and highlight the need to improve QoL in these patients.