In this study, biopolymer composites were developed using poly(lactic acid) (PLA) as a polymer matrix. Various agricultural by-products, including flax seed meal, rapeseed straw, and mustard seed meal, were added as a reinforcement. The research aimed to provide insight into the valorization of cheap, readily available residues generated in the agricultural industry and assess the mechanical properties of composites prepared using them. The experimental fabrication was conducted by compounding PLA with agro-waste particles in 10 and 20 wt% concentrations. These components were melt mixed with a twin-screw extruder and injection molded into standardized forms. The resulting fabricated composites were tested for tensile and flexural mechanical properties and hardness. Through scanning electron microscopy, images of the natural particles were taken to better understand their structure, geometry, and possible ways of interaction between them and the PLA matrix. The results of quasi-static mechanical tests suggest that using agricultural by-products can effectively improve Young's modulus and flexural modulus of PLA but at the cost of tensile and flexural strength, which decreased with the by-products' introduction. Of the three agro-waste options, rapeseed straw emerged as the superior choice because it only marginally reduced the mechanical strength of the PLA and enhanced its stiffness the most. Hardness was the least affected property, test results showed that the added fillers did not substantially change the polymer matrix's hardness.