The growing marine renewable energy sector has led to a demand for increasingly compliant mooring systems. In response, several innovative mooring tethers have been proposed demonstrating potential customisation to the stiffness profile and reduced peak mooring loads. Many of these novel systems utilise materials in a unique application within the challenging marine environment and their long term durability remains to be proven.This paper presents a multifaceted investigation into the durability of a novel polyester mooring tether with an elastomeric core. Laboratory based functionality tests are repeated on tether assemblies following a 6 month sea deployment. Results show a 45% average increase in dynamic axial stiffness. This is supported by high tension laboratory based fatigue endurance tests showing a peak increase in dynamic axial stiffness of 42%. Sub-component material tests on the core elastomer support the assembly tests, separately demonstrating that certain aspects of tether operation lead to increased material sample stiffness. The average increase in material radial compressive stiffness is 22% and 15% as a result of marine ageing and repeated mechanical compression respectively; these are the first results of this type to be published.The performance durability characterisation of the tether establishes the mooring design envelope for long-term deployment. This characterisation is crucial to ensure reliable and effective integration of novel mooring systems into offshore engineering projects.