Background: Advanced mycosis fungoides (MF) and Sézary syndrome (SS) are rare, aggressive cutaneous T-cell lymphomas that may be difficult to treat. Mogamulizumab is a recent monoclonal antibody targeting the CCR4 receptor expressed on the surface of Sézary cells. It can be prescribed in MF/SS stages III to IV in the second line after systemic therapy or in stages IB-II after two unsuccessful systemic therapies. We lack data on long-term efficiency and potential side effects in real-life conditions. Our study aims to determine efficacy considering the median PFS of advanced CTCL with mogamulizumab. Secondary objectives were to consider tolerance and estimate delay until side effects appeared. Methods: Data on patients with advanced cutaneous T-cell lymphomas were collected since French Authorization, in six French university hospitals. Patients were followed until they stopped mogamulizumab because of relapse or toxicity. For those still treated by mogamulizumab, the end point was 1 September 2021. We excluded 3 patients as they had already been included in the MAVORIC study and data was not available. Results: The median time of follow-up was 11.6 months. Of the 21 patients included, we reported four full-response patients, eight in partial response, one in stability, three in progression, and five were deceased. One patient had visceral progression, and seven had new lymphadenopathy. Progression-free survival was estimated at 22 months. Twenty patients presented adverse events, of which 10 were severe, i.e., grade III-IV. The median time between the introduction of mogamulizumab and the first adverse event was 21 days. Conclusions: Our study suggests that mogamulizumab can give patients with advanced refractory CTCL a consequent PFS, estimated at 22 months. The long-term safety of mogamulizumab was determined to be acceptable since we reported few grade III–IV AEs, comparable with other studies. No other study using real-life data has been performed to investigate the AEs of mogamulizumab.