A cost effective and ecofriendly alternative for commercially available microwave substrates is proposed in the present study. A polymer composite based on high-density polyethylene (HDPE) filled with garnet mineral was fabricated using sigma-blend technique with 0.1-0.5-volume fraction of filler. The atomic force microscopic images indicated the surface roughness of the substrate is very low and has the required degree of surface finish to avoid conduction loss. For 0.5 vol fraction of garnet, the composite showed a dielectric constant (epsilon(r)) = 5.7 and dielectric loss (tan delta) = 0.00185 at 5 GHz. Moreover, coefficient of thermal expansion (CTE), thermal conductivity (k(c)) and tensile strength are 114 ppm/degrees C, 1.3 Wm(-1)K(-1) and 14.3 MPa respectively for filler volume fraction, V-f = 0.5. The dielectric, thermal, moisture absorption and mechanical properties indicate that the garnet/HDPE composites are ideal candidate for cost effective and sustainable microwave substrates.