Polymer–ceramic composites have been prepared by dispersing Ca4La6(SiO4)4(PO4)2O2 (CLSP) ceramic filler in high density polyethylene (HDPE) matrix through melt mixing. Scanning electron micrographs reveal the extent of filler dispersion. The dielectric properties at 1 MHz and 5 GHz have been investigated as a function of filler content. The relative permittivity increases with filler loading, maintaining a low dielectric loss. The composite with highest filler loading of 0.4 volume fraction shows a relative permittivity of 5.1 and dielectric loss of 2.3 × 10−3 at 5 GHz. Experimentally observed values of relative permittivity at 5 GHz have been compared with the values calculated using various theoretical models. Both the coefficient of linear thermal expansion and tensile strength have been observed to decrease with filler loading, reaching a minimum value of 117 ppm/ °C and 20.7 MPa, respectively, at 0.4 volume fraction of filler. The composite with maximum filler loading of 0.4 volume fraction shows the highest thermal conductivity (TC) and is 1.2 W m−1 K−1.