Increasing solid waste has stimulated growth in the development of biodegradable plastics from sustainable resources. Novatein thermoplastic protein (NTP®) is produced from the by-products of the meat processing industry and has comparable tensile strength and modulus to that of linear low density polyethylene (LLDPE). However, its impact strength is very low in comparison to other commodity plastics. This study produced blends of NTP® and functionalised polyethylene (Lotader AX8900, maleic anhydride grafted polyethylene and Surlyn 9320) through reactive extrusion. Mechanical properties of the blends were assessed using tensile and impact testing, while morphology was assessed using electron and optical microscopy. Impact strength improved over 400%, depending on the specific polyethylene functionality and blend composition. It was established that phase separation occurred in all blends to varying degrees, depending on composition, and that the morphology of the blends was co-continuous. The inclusion of urea in the NTP® formulation proved detrimental to mechanical properties due to the preferential reaction of urea and reactive functionalities on the polyethylene, rather than reactions between protein functional groups and functionalised polyethylene. Higher extrusion temperatures caused excessive moisture loss from the protein phase leading to a very brittle material.