Paulownia wood (PW) flour was evaluated as a reinforcement for thermoplastic composites. Composites of high-density polyethylene in pellet form (HDPE), 25% by weight of PW, and either 0% or 5% by weight of maleated polyethylene pellets (MAPE), were produced by twin screw compounding followed by injection molding. Formulations of PW flour composed of specific particle sizes (≤590 to ≤75 µm) were also compared. Molded test composites were evaluated for their tensile, flexural, impact, and thermal properties. Composites made with PW and MAPE had significantly improved tensile and flexural properties compared to neat HDPE. The impact strength of all composites using MAPE was 30% improved over HDPE. Benchmarking PW composites to similar preparations of pine wood flour composites demonstrated that PW can produce a comparable and in some cases a superior bio-fiber composite. The effect of environmental exposure was examined by soaking tensile bars of HDPE-PW blends in distilled water for 28 days to observe changes in their physical and mechanical properties. Finally, differential scanning calorimetery and thermogravimetric analysis were conducted on PW composites to evaluate their thermal properties and the implications these may have on selecting processing conditions for the bio-fiber reinforcements.