The addition of nanoparticles to a polymermatrix is a well-known process to improve the mechanicalproperties of polymers. Many studies of mechanical reinforcementin polymer nanocomposites (PNCs) focus on rubberymatrices; however, much less effort concentrates on thefactors controlling the mechanical performance of thetechnologically important glassy PNCs. This paper presentsa study of the effect of the polymer molecular weight (MW)on the overall mechanical properties of glassy PNCs withattractive interaction by using Brillouin light scattering. Wefound that the mechanical moduli (bulk and shear) have anonmonotonic dependence on MW that cannot be predictedby simple rule of mixtures. The moduli increase with increasing MW up to 100 kg/mol followed by a drop at higher MW. Wedemonstrate that the change in the mechanical properties of PNCs can be associated with the properties of the interfacialpolymer layer. The latter depend on the interfacial chain packing and stretching, as well as polymer bridging,which vary differently with the MW of the polymer. These competing contributions lead to the observed nonmonotonicvariations of the glassy PNC moduli with MW. Our work provides a simple, cost-effective, and efficient way to control themechanical properties of glassy PNCs by tuning the polymer chain length. Our finding can be beneficial for the rational designof PNCs with desired mechanical performance.