The environmental impact of petroleum-derived materials has intensified the search for sustainable and biodegradable alternatives consistent with circular economy principles. Mycelium-based composites (MBCs) offer a biological route for transforming low-value lignocellulosic residues into functional materials. This review analyses fungal species, fungal biology, substrates, fabrication strategies, material properties, applications and production costs using the interaction among fungal species, substrate and processing as its organising principle. The literature survey records 126 fungal species tested or screened for composite production. The influence of substrates, cultivation conditions, fabrication methods and post-processing on material properties is critically evaluated. Recent advances in biofabrication and performance improvement are discussed in relation to their technological maturity and cost effectiveness. Applications in packaging, construction, insulation, furniture and textiles, which range from laboratory proofs of concept to commercial products, illustrate the versatility of MBCs, while variability, process standardisation, large-scale manufacturing and performance optimisation remain challenges. The environmental benefits of MBCs are shown to depend more on production steps than on their biological origin alone. The review identifies research priorities for making production more scalable and economically competitive, including species selection, cultivation optimisation and the integration of biological knowledge with material processing. These directions provide a basis for advancing fungal materials within sustainable manufacturing and circular resource use.