There is a growing interest in more sustainable fibre reinforced polymer (FRP) composite materials for civil engineering applications. However, the adoption of such composites requires assessing their long-term durability performance in harsh environmental conditions, as civil engineering structures are designed for long service lives, typically 50 or 100 years. This paper describes an experimental study which aimed at assessing the hygrothermal durability of a glass-FRP (GFRP) composite produced with a partially bio-based unsaturated polyester resin (UPR) by vacuum infusion. The bio-based UPR was developed (in-house) incorporating renewably sourced monomers, namely fumaric acid, 1,3-propanediol, and isosorbide. Additionally, styrene content was reduced (~50% relative to typical UPRs) by partial replacement with 2-hydroxyethyl methacrylate. The GFRP composite was exposed to water immersion for up to 720 days at three different isothermal conditions, at 20 °C, 35 °C, and 50 °C. The effects of water immersion at different temperatures were assessed after 30, 90, 180, 360, and 720 days of exposure through mechanical testing (tension, compression, and interlaminar shear) and thermomechanical characterisation using dynamic mechanical analysis (DMA). As a benchmark, a “conventional” GFRP composite with similar fibre architecture and an oil-based UPR was also assessed under identical conditions. Although the initial properties of both bio- and oil-based GFRP composites were comparable, the hygrothermal durability performance of the bio-GFRP composite was inferior. After 24 months of hygrothermal exposure, property retentions for the bio-based GFRP ranged between ~30% (compressive and interlaminar shear strengths) to ~60% (tensile strength and glass transition temperature), while the oil-based GFRP exhibited retentions of around 60% to 70%. The results suggest the occurrence of irreversible damage in the bio-based GFRP composite during the early stages of water immersion (within 3 months), irrespective of the thermal condition. This degradation was followed by the stabilization of mechanical and thermomechanical properties during the later stages of hygrothermal ageing. Hydrolysis, loss of fibre-matrix adhesion, and matrix plasticisation should be the dominant damage mechanisms causing hygrothermal degradation.