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In the competitive market of plastic fillers, inexpensive and reliable materials are always sought after. Using a method of thermal conversion, called pyrolysis, a potential contender was created from a plant biomass known as soybean hulls (SBH). The SBH are a byproduct of the soybean farming industry and represent an abundant and inexpensive feedstock. Thermal conversion of the SBH material gives rise to a lightweight carbon-rich filler called pyrolyzed soybean hulls (PSBH). Two separate lots, lot A and lot B, were created with lot A corresponding to SBH pyrolyzed at 450°C (PSBH-A) and lot B corresponding to SBH pyrolyzed at 500°C (PSBH-B). Both lots of PSBH were also milled to reduce their particle size and tested against the as-received PSBH fillers. These milled materials were designated as ground soybean hulls (GSBH).Two different polyolefins, linear low-density polyethylene (LLDPE) and polypropylene (PP), were used for this study. The PSBH fillers were added to the polyolefins in weight percentages of 10, 20, 30, 40, and 50 percent with the resulting plastic/PSBH composites being tested for their mechanical, thermal, and water absorption properties. In general, the addition of filler increased the maximum stress of LLDPE/PSBH composites while reducing maximum stress of PP/PSBH composites. The strain at maximum stress was reduced with increasing amounts of PSBH filler for all composites. Modulus of elasticity generally increased with increasing filler amount. For thermal properties, the addition of PSBH filler increases the heat distortion temperature, increases the thermal decomposition temperature, and reduces the heat of fusion of the composites compared to the neat polyolefins. The liquid absorption and thickness swelling in the materials were small overall but did increase with increasing amounts of PSBH filler and with the time spent submerged in liquid. Milling the PSBH material into GSBH generally had small effects on the various material properties tested and led to easier mixing and a smoother finish on the surface of processed samples. The differences observed between lot A and lot B composites was often small or even negligible.