By means of the integration embedding scheme, a model is developed for the complex dielectric permittivity and magnetic permeability of polymers reinforced with ferromagnetic particles. The model takes into account the aspect ratio of particles and their aggregation into clusters. The governing equations involve six material parameters that are found by matching the experimental data on the real and imaginary parts of the dielectric permittivity and those of the magnetic permeability (four curves are fitted simultaneously for each material) of polymers filled with spherical and flake-shaped micro- and nanoparticles. Good agreement is demonstrated between the results of simulation and observations on polymers reinforced with carbonyl iron, cobalt, and FeCoNi alloy particles at microwave frequencies in the X-band of the electromagnetic spectrum. The model is applied to evaluate the effectiveness of electromagnetic interference shielding in the transmittance mode by polymer absorbers with spherical and flake-shaped particles. Numerical analysis shows that for the same effectiveness of shielding, reinforcement of polymers with flakes (instead of microspheres) allows the volume fraction of filler to be reduced by a factor of 2 to 3.