Medium-chain-length poly(hydroxyalkanoate)s (PHAmcl) are biodegradable and renewable biopolymers with elastomeric qualities. Here we report on the preparation and characterization of composite materials using thermally reduced graphene (TRG) nanoparticles as filler with three PHA mcl polymers. The matrices vary with respect to chain packing length, capacity for noncovalent bonding with the TRG surface, and the presence of covalent cross-linking. Results show that the addition of up to 2.5 vol % TRG to PHAmcl increases the melting temperature by 1-3 °C, the modulus by 200-590%, and the electrical conductivity by >7 orders of magnitude. Additionally, we use rheology and microscopy to characterize the composites. We discuss our results in light of polymer entanglement theory and the effects of polymer structure, filler loading volume, and the role of graphene-polymer interfacial forces. We extend our discussion by comparing the modulus enhancements of PHAmcl composites to those reported in other studies in which layered carbon nanofillers are combined with structurally related biopolyesters including: polylactide, polylactide-co-polyglycolide, polycaprolactone, and two other PHA copolymers.