Mooring systems are under a cyclic loading process caused by the randomness of metocean conditions, which could lead to a fatigue failure of the station keeping system. The present paper presents an innovative methodology for the assessment of floating offshore wind turbine mooring system fatigue considering the full lifetime of the structure. The method integrates the impact of the life cycle metocean conditions over the dynamic performance of the platform thanks to coupled numerical models, selection and non-linear data interpolation techniques and commonly accepted fatigue approaches. One of the benefits of using this methodology is that there are no uncertainties due to the selection of a reduced set of sea states. The methodology is applied to a set of moorings with different properties in the DeepCwind platform to evaluate the solution which offers the best compromise between size and fatigue damage. Results show that the best long-term mooring behaviour is achieved with a weight of approximately 300 kg/m. A comparison is conducted between the fatigue damage obtained through the life-cycle method and conventional methods. The mean differences observed between the standard and the new method proposed are between 13% and 49% depending on the use of the S–N or T-N curves.