The prognosis of patients with mycosis fungoides is variable. As the current literature is scarce and shows mixed results this study investigates the incidence of other primary malignancies in mycosis fungoides patients. A retrospective, nationwide, population- based cohort study was performed with patients with mycosis fungoides between 2000 and 2020 in The Netherlands. All histopathology reports were requested from the Nationwide Network and Registry of Histo- and Cytopathology and screened for other primary malignancies. Lifelong incidence rates were used to compare the incidence of malignancies in mycosis fungoides patients and the general population. In total 1,024 patients were included with a mean follow-up of 10 years (SD 6). A total of 294 cases of other primary malignancies were found with 29% of the mycosis fungoides patients developing at least 1 other primary malignancy. Only cutaneous (odds ratio [OR] 2.54; CI 2.0–3.2) and haematological malignancies (OR 2.62; CI 2.00–3.42) had a statistically significant higher incidence than the Dutch population overall. Mycosis fungoides patients have a significantly increased risk of developing melanomas (OR 2.76; CI 2.11–3.59) and cutaneous squamous cell carcinomas mycosis fungoides (OR 2.34; CI 1.58–3.45). This study shows no association between mycosis fungoides and other solid organ tumours; however, such patients are significantly at risk of developing other haematological and cutaneous malignancies. Clinicians should be aware of this increased risk.