Hemp hurd can be a valuable resource to the biocomposite industry as a sustainable source of biofiber and biofiller. A comprehensive study to understand the impact of biocarbon from agriculture biomass, i.e., hemp and switchgrass, on the physical, thermal, and mechanical properties is missing, which the current work aims to fill. The knowledge will support the development of biocomposites. Biocarbon from pyrolysis was produced at 450, 550, and 650 °C. Composite samples were prepared with 10 wt.%, 15%, and 20 wt.% of biocarbon fillers sized below 50 microns to below 100 microns. At the optimized conditions, the values of water absorptivity due to hemp biocarbon and switchgrass biocarbon were 0.72 x 10-6 g. m-2. s-1/2 and 0.73 x 10-6 g. m-2. s-1/2, respectively, lower by 70% from the unfilled hemp fiber-reinforced composites. The water absorption improved the tensile strength and impeded the flexural strength. The energy at tensile rupture increased with the particle size; in contrast, decreased with the filler loading. Maximum tensile strength of 840.75 MPa and 817.02 MPa was 10 wt.% biocarbon of 50 microns obtained at 650 °C from switchgrass and hemp stalk, respectively. The pyrolysis temperature, particle size, and filler positively affected the composite samples' flexural strength. The impact strength of the composite samples decreased by almost 63% when the filler loading was doubled. The composites’ hardness significantly increases with the increase in filler loading. The thermal conductivity increased with filler-loading and particle size—the biocomposite samples with 20 wt.% filler loading of 100 µm particle size of the biocarbon obtained at 650 °C showed the maximum flame time and thermal conductivity in biocarbon-filled composites. The hemp composites with 1300 kg/m3 density and 800 MPa tensile strength overlap with the properties of some ceramics (Si3N4) and some metal alloys (magnesium alloys, aluminum alloys and silicon carbide) in the property chart. The hemp composites offer better tensile strength than the polymers and elastomers (PEEK, PC, PA, PMMA, PET silicone elastomers), concrete, woods, and rigid and flexible polymer foam and cork. Based on the material property chart of strength vs. toughness, the composite samples lie in the metals and alloys region with tough and strong behaviour better than the ceramics, glasses, wood products, polymers, and rubbers. These findings suggest potential hemp composites usage for mechanical load-bearing and thermal insulating applications.