The manufacture of natural fiber and core preforms for biocomposite sandwich structures that bound together with fungal mycelium-based polymer is investigated. The complete manufacturing process involves: (1) cutting individual textile plies; (2) impregnating multi-ply layups with natural glue conducive to mycelium growth; (3) simultaneously forming, sterilizing and setting impregnated skins; (4) filling formed skins with mycelium-laden agri-waste; (5) allowing mycelium to colonize and bind together core substrate and skins into a unitized preform; (6) high temperature drying that also inactivates fungus; and (7) infusing skins with bioresin using resin transfer molding. Aspects of steps 3–6 related to the preform shells and sandwich structure are the main focus of this paper. Three-point bending tests are performed on dry, natural glue-bonded, four-ply specimens in a full-factorial experimental design, and test results are analyzed by analysis of variance (ANOVA) to assess process parameter effects and sensitivities along with environmental condition effects. New specimens are then made using the optimized process and tested for beam bending in creep within an environmental chamber that mimics the actual mycelium growth environment for three days. Two- and six-ply specimens loaded to provide identical maximum tensile stress in flexure are tested, and useful conclusions are drawn based on all creep test results. Finally, preforms in the shape of a viable commercial product are filled with mycelium-inoculated substrate, grown and dried, and part quality is evaluated based on the amount of skin ingrowth and deviation between the measured and desired shapes.