The long-term performance of any novel material in its working environment is an intriguing area of research. The combined effect of the moisture content (relative humidity) and temperature of the environment degrades the mechanical properties of polymer composites. In the first part of this manuscript, polyurea–polyhedral oligomeric silsesquioxane (POSS) nanocomposites are subjected to continuous hygrothermal cycling. The effects of prolonged hygrothermal aging on the mechanical performance of the composites are evaluated in terms of the change in tensile strength and elongation at failure. Aged composites are subjected to FTIR spectroscopy to study the chemical changes resulting from the hydrolysis of polyurea. In the second part, attempts are made to study the effect of the addition of POSS nanoreinforcement on the mechanical properties of the resulting composite materials using molecular dynamics (MD) simulations. Results obtained here show poor agreement between MD simulation results and results obtained using dynamic mechanical analysis studies. However, qualitatively, both simulation and experimental results exhibit similar effects due to addition of POSS nanoparticles on the mechanical properties of polyurea–POSS nanocomposites.