Due to many advantages, polymer composite insulators have been extensively used for high-voltage (HV) transmission lines and substation insulations. The in-service operation, various environmental and electrical stresses degrade their mechanical and thermal characteristics. In this study, nine thousand-hour (9000 h) multi-stress (HV, heat, acid rain, salt fog, ultraviolet (UV) radiation, and humidity) accelerated lab-weathering evaluation of alumina-trihydrate (ATH) and silicon dioxide (SiO₂) filled silicone rubber (SiR) composites were utilized. Moreover, to quantify the influence of multiple stresses over 9000 h lab-aging, the tensile strength, elongation at break, hardness, and thermal properties were evaluated and compared with the characterization results of neat (un-aged) composites. Winter and summer-aging cycles were designed in accordance with the actual service environment of Islamabad (Pakistan). Mechanical results of SiR blends showed a decrease in the tensile strength and the elongation at break (EAB), whereas the hardness increased over 9000 h lab-aging. The maximum deviation of ∼57.1% in tensile strength was found for hybrid samples (micro-ATH+ nano-Silica blend: SMNC), whereas the minimum change of ∼25.73% was exhibited by micro-silica-filled SiR specimen SMC3. Compared to neat blends, the maximum variation in EAB was ∼61% for a neat sample (SiR), whereas minimum change was noticed for SMC2 (of ∼31%) over 9000 h lab-aging. Additionally, after 9000 h lab-aging, the maximum (of ∼79.6%) and minimum (of ∼24.4%) variation in hardness was found for hybrid and SiR samples, respectively. Moreover, thermogravimetric (TGA) analysis showed that relative to neat samples, the thermal stability of aged specimens was decreased over weathering. Among aged specimens, only ATH filled samples (SMC1, SMC2) exhibited superior performance for a given temperature range (from 0°C to 900°C) by leaving a higher residual weight of ∼68.6% for SMC2. Hence to simulate and quantify the influence of environmental stresses over insulant performance, accelerated lab weathering can be adopted as an efficient tool.