Recent advances in the field of biomaterials and an ever-growing need to curb the alarming rate of pollution levels have led to the utilization of biodegradable waste to fabricate sustainable materials with tunable properties. The current study investigated the growth kinetics and morphology of Pleurotus ostreatus (P. ostreatus) mycelium grown on different agricultural wastes such as wheat bran, sugarcane, sawdust, and the mixture of these substrates. Further, it delineated the fabrication process of biodegradable “bioblocks” from such agricultural waste using a green synthesis approach and mycelium P. ostreatus as a natural adhesive material. The fabricated bioblocks showed excellent thermal stability, hydrophobic properties, and mechanical strength. The compressive strength of these bioblocks was approximately 6.0–7.5 N/mm2, which is 5–6 times higher than that of the routinely used polystyrene packaging material. These properties of the bioblocks render them fit to replace the non-biodegradable materials that are commonly used in packaging applications, wall paneling, and filtration of toxic wastes.