In this study, synthetic graphite and graphene were used as thermally conductive fillers for polyester-based composites. Orthophthalic polyester resin, methyl ethyl ketone peroxide and cobalt octoate were used as the polymer matrix, catalyst and accelerator, respectively. Hybrid carbon (C) fillers at different weight fractions (up to 45 wt.%) were added into the polymer matrix by using an ultrasonic mixer. Composite test specimens were prepared by using the doctor blade thin-film fabrication method. The in-plane and through-plane thermal conductivity values of the composites were determined by using a Xenon Flash instrument. Furthermore, the change in the morphological structure of the carbon-filled composites was studied in detail by scanning electron microscopy. Mechanical properties were studied by performing tensile tests, and the electrical conductivity of composites was measured using an LCR meter. Significant enhancement of the thermal conductivity by way of hybrid carbon filler incorporation was obtained for polyester-based composites.