In this study, the influence of resin crosslink density on thermo-mechanical properties and fatigue behavior of polyurethane resins and glass fiber reinforced polyurethane (PU-GFRP) composites were investigated. This material is a candidate for renewable energy applications like rotor blades in wind turbines. The thermo-mechanical behavior and morphologies of three different PU resin systems were analyzed using dynamic mechanical analysis (DMA) and atomic force microscopy (AFM). Results show that the system with low crosslink density exhibits earlier crack initiation but higher strains before failure, indicating initially a more brittle behavior but greater polymer chain reorientation capacity. This phenomenon was correlated to the different phase morphology and the hybrid nature of the investigated PU resins. Residual strength analysis showed minimum influence on tensile strength, and a slight modulus decrease after extensive creep tests. This highlights the importance of fiber-matrix interaction on the composite fatigue failure. The static mechanical properties of the composites and SN fatigue tests allowed assessing the fatigue performance and lifetime analysis under specific loading conditions. Highly crosslinked systems show a significantly longer lifetime compared to the medium crosslinked system. In conclusion, the study highlights the importance of understanding resin crosslink density’s impact on fatigue behavior in GFRP composites for better material and part design.