Materials with high dielectric permittivity are highly desirable in the electronics industry. One avenue for enhancing the permittivity of standard metal oxide and ceramic dielectrics is to incorporate nanoscale Ag and Au inclusions in the material. Given the small scale of modern day devices, these inclusions will necessarily be up to a few nanometers in size. We develop methodology by which polarization in nanocomposites with molecular-scale inclusions can be obtained from first-principles calculations, and partitioned into inclusion and matrix contributions. The methodology is applied to a model Ag8/MgO nanocomposite. A 4% volume loading of Ag8 nanoparticles leads to a 30% enhancement of the dielectric permittivity. The enhancement arises from both the electronic polarization of the nanoparticle and the additional polarization of matrix ions in the interfacial region.