Fiber-reinforced wood-plastic composites (WPCs) have garnered significant interest in several sectors, such as construction, automotive, and packaging, owing to their superior combination of strength, rigidity, and resilience to environmental deterioration. This research examines the mechanical and creep characteristics of several polymer matrices and their composites reinforced with natural and glass fibers, emphasizing the necessity of keeping processing temperatures below 200 °C to avert fiber breakdown. Polypropylene (PP), polystyrene (PS), and two types of polylactic acid (PLA) with varying crystallinity and viscosity were compounded with compatibilizers, including maleic anhydride grafted polymers, to improve interfacial bonding. Comprehensive drying processes and reactive extrusion methods guaranteed maximum moisture regulation and material uniformity. Tensile and flexural creep experiments performed under regulated humidity and temperature settings shown notable disparities in deformation and dimensional stability between pure polymers and their fiber-reinforced variants. PLA-based composites exhibited enhanced creep resistance compared to PP, which had the highest deformation due to its reduced molecular weight and tensile modulus. These findings offer significant insights into the selection and processing of bio-based and synthetic polymer composites for load-bearing applications, highlighting the essential equilibrium between mechanical characteristics and long-term dimensional stability.